Renaud Roussillon, a 41-year-old architect, was waiting for a train at the Grande Arche de la Défense station in Paris' busiest business district at 11.20am one morning in April 2009 when he was suddenly pushed from behind.
Roussillon was trapped under the metro train for an hour and a half while emergency services tried to free him. After the attack one of his legs had to be amputated.
The accused is a homeless man who was waiting on the platform. He admitted the attack but said he was not in his right mind that day.
"I lost control and pushed him with both hands, seized by anger, irritation," he said. "I'm really sorry."
Roussillon left the courtroom when closed circuit footage was shown of the moment when he was pushed.
He still finds it difficult to understand why he was attacked.
"We didn't look at each other or speak to each other, it was just that he couldn't stand the sight of me," he told radio station France Info.
"I didn't have an iPhone or any sign of wealth like a nice watch," he said. "Since the 10th of April 2009 I feel like I've been given a life sentence without being given any choice."
The man's attacker has been given psychological tests to check his state of mind and a verdict is expected later on Friday.