PURPOSE: To reduce the cost to form marks of a key by irradiating mark forming parts of the key with a laser beam in an oxygen and nitrogen atmosphere and forming titanium oxide or titanium nitride on the irradiation parts to discolor the parts.

CONSTITUTION: The marks 5 are formed on the surface of the key 1 whose prescribed surface is made of pure titanium or a titanium alloy. The parts where the marks 5 of the surface of the key 1 are formed are then irradiated with the laser beam 2 in gas 6 containing oxygen and nitrogen. The titanium oxide or titanium nitride is then formed on the laser beam irradiation parts 1a and 1c which are irradiated with the laser beam 2 and the parts are discolored to form the marks 5. By this method, the marks are formed on the prescribed parts of the key.