An immigration law firm in Yokohama is refusing to give a passport back to a Filipino woman under an employment contract they signed earlier this year, preventing her from seeking a new job or returning home, she said Monday.

It is illegal for employers to keep passports of foreign nationals who live in Japan as technical trainees. But the country has no penal regulations concerning those of other foreign nationals, with only the labor ministry's guidelines advising employers not to keep them.

The case of the Filipino woman suggests this legal pitfall could potentially put foreign workers in a precarious position, even at a time when Japan is opening its doors wider to them, experts say.