PURPOSE: To achieve simple, sure and efficient stress relief, by bringing a cooling metal having high thermal conduction into contact with a projected part produced by heating, in a stress relief method for thin plate construction.

CONSTITUTION: A tip 1a of a cylindrical body 1 is abutted on a projected part caused through heating in dot or linear shape and trigger 11 is drawn. A lock pin 8 moves downward and a cooling metal 2 is brought in hitting contact with the projection caused by heating with prescribed force obtained by a spring 7. A valve 4 provided at a port 1b is opened through the striking of the cooling metal 2 and a coolant flows in a cooling hole 2e and is blown to the heating part. Thus, the heated projection is made flat and the contact area with the cooling metal 2 is increased. The cooling speed is further higher through the blowing of the coolant. Next, when a grip 2c is drawn, the cooling metal 2 moves backward, the valve 4 is closed, the trigger 11 moves forward with a return spring 16 and the pin 8 is projected in the cylindrical body 1 to lock the cooling metal 2.