Constitution of Japan, Article 18, prohibition of forced labor. So "illegal" isn't the right word.
While damages are theoretically possible for leaving a fixed term employment contract early (with an exception after one year has passed), I'd be very interested in the precedent you're talking of, regarding an actual case of a contract employee being sued for quitting early.
Only case I found was the K's International case, 1992, where an employee quit after 4 days, and the employer sued and was awarded damages (amount unknown) due to the disruption it caused. I couldn't find any further details though.
While damages are theoretically possible for leaving a fixed term employment contract early (with an exception after one year has passed), I'd be very interested in the precedent you're talking of, regarding an actual case of a contract employee being sued for quitting early.
Only case I found was the K's International case, 1992, where an employee quit after 4 days, and the employer sued and was awarded damages (amount unknown) due to the disruption it caused. I couldn't find any further details though.