Gunpei Yokoi, a brilliant tinkerer who designed the Game Boy and other products that helped transform Nintendo from a sleepy manufacturer of playing cards into a worldwide colossus in video games, was killed in a traffic accident in Japan on Saturday, the company said.

Mr. Yokoi, 56, was a passenger in a car that hit a truck from behind on an expressway in Ishikawa Prefecture, about 215 miles northwest of Tokyo, according to a report in Yomiuri Shimbun, a major Japanese daily newspaper. After Mr. Yokoi and the driver of the car got out to examine the damage, another car struck them, killing Mr. Yokoi and injuring the other man.

”He was a creative genius,” Howard Lincoln, chairman of Nintendo of America, said in an interview. ”He was a warm, friendly and outgoing gentleman who was universally loved by all Nintendo employees he came in contact with.”

When Mr. Yokoi left Nintendo in August 1996 to start his own toy company, the news contributed to a run on Nintendo’s stock that forced the Tokyo Stock Exchange to halt trading. However, the main reason for the heavy stock selling that day was a newspaper report that profits would fall.

Mr. Yokoi was the first head of research and development and the first game designer at Nintendo. His greatest accomplishment was the hand-held Game Boy. About 55 million have been sold worldwide since its introduction in 1989.

Mr. Yokoi grew up in Kyoto and received a college degree in electronics, according to ”Game Over,” a book about Nintendo by David Sheff. He was hired in 1965 to maintain the assembly line machines that made playing cards at Nintendo, a family-owned company based in Kyoto that had been in that business since the late 1800’s.

At that time, playing cards imported from the United States were beginning to hurt Nintendo’s business, so Hiroshi Yamauchi, the company’s driven president and the great- grandson of the founder, diversified into other games. Around 1970, according to Mr. Sheff’s book, Mr. Yamauchi called Mr. Yokoi into his office and asked him to make something the company could sell that Christmas.

”What should I make?” Mr. Yokoi asked.

”Something great,” Mr. Yamauchi replied.

A tinkerer, Mr. Yokoi had devised for his own amusement a wooden device that could grip things at a distance, a sort of extension of the arm. The Ultra Hand became Nintendo’s first toy and 1.2 million units were sold, according to the book.

Mr. Yokoi went on to devise many hits including the ”love tester,” which ”measured” the level of affection between a boy and girl who held hands and touched the device, and the ”laser clay range,” in which abandoned bowling alleys were turned into shooting galleries where people fired at clay pigeons with light-beam guns.
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As Nintendo grew and moved into electronic games, it was split into three research and development departments that competed against one another. Mr. Yokoi, head of department No. 1, tended to focus on small, hand-held games, rather than on the company’s home video game machines.

His first hand-held game was the Game & Watch, which came out in 1980 and was about the size of the calculators proliferating at the time. It had a simple video game on a liquid crystal screen and a clock in the corner.

That led to the Game Boy, also a hand-held game but one that could play different games stored on cartridges.

Mr. Yokoi’s track record was broken by his final product for Nintendo, the Virtual Boy, which used virtual reality technology. That product, which required users to look into a viewer, failed to catch on. ”He took it quite personally when it was not successful,” Mr. Lincoln said.

In 1996 Mr. Yokoi left to start his own toy company in Kyoto. Since executives in Japan tend to stay with one company for life, the news caused speculation that Nintendo had problems. But Mr. Lincoln said Mr. Yokoi kept a close relationship with Nintendo.