Lee told reporters in Tokyo he came up with the idea while watching a fly climb a wall and wanted to create a superhero with the same abilities.

"Somehow calling him 'flyman' didn't sound dramatic enough. What else could he be? Mosquito man? Then I said: Spiderman. And it sounded so dramatic," said Lee.

Lee decided to make his new creation a teenager and give him "many personal problems".

"He won't have enough money, he lives with his aunt who is ill and needs medicine and he has to take care of her. And he's also got to fight the bad guys," he said.

He took the idea to his publisher who said: "Stan, that is the worst idea I have ever heard."

Superheroes have to be adults and cannot have any problems -- that's why they are superheroes, said his boss.

And another thing: "People hate spiders so you can't call a hero Spiderman!" 

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Lee was nevertheless allowed to slip a Spiderman story into the last edition of a book and it became an instant hit.

"Just for fun, to get it out of my system, I put Spiderman in that last edition and forgot about it," he said.

"A month later, after all the sales figures were in, my boss came running into my office and said: 'Remember that character Spiderman we both liked so much, let's make him a regular feature'."

"And that's how Spidey was born," said Lee to applause and laughter.

The energetic 94-year-old, who admitted to driving around the streets of Tokyo in a "Super Mario" drag car late the previous evening, was asked how he kept so active.

"Being busy is the best medicine there is. As long as you're not busy being a villain," he said.

As for how he came up with the idea of Spiderman, the answer was simple: "I'm very lucky. I'm brilliant."

Lee was in Japan to promote the second edition of the Tokyo Comic Convention that opens on Friday.