Hidetsugu Aneha, 48, has told the Japanese press that he fiddled the numbers on at least 21 housing projects because he was under pressure from developers to cut costs.
He is now facing legal investigations relating to his admission of responsibility.
Aneha, who is fully qualified and certified, has warned that the buildings, which are now inhabited, could be completely destroyed by a major earthquake.
Japan is sited on one of the most active tectonic faultlines in the world and has stringent structural regulations.
But when questioned by journalists from local paper, The Daily Yomiuri, the architect admitted his wrong-doing, saying: 'If you look at the numbers, its pretty obvious'.
'I wasn't hard up for work or for money,' he told the newspaper, 'but after I started doctoring the information, my actual workload increased and as I tried to deal with that, I completely forgot about the people.
'In a worst-case scenario, they [the buildings] would be completely destroyed. Even if that didn't happen, the pillars might bend or the buildings could collapse,' Aneha said. 'People would get hurt'.