PURPOSE: To effectively execute with lap welding the galvanized steel plate by forming the optical laser to two kinds of convergent lasers separated of the high and low energy density at the same time.

CONSTITUTION: The laser light is divided to the 1st convergent laser of low energy density and the 2nd convergent laser of high energy density with the optical converging means 21 against the lapped part of the galvanized steel sheet the former radiates the lapped part of the steel sheet and the galvanized zinc is evaporated and dispersed, and the later melts the steel sheet and joins it. Therefore, on the laser lap welding of the galvanized steel plate, the welding can be executed effectively and well without the fault of blow holes, etc.