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.