To provide a lap welding machine where the rolling reduction pressure of planishing rolls is set easily and the welding part of lapped steel plates is rolled surely in a requiring thickness without generating marks like a sandal insect.

In a lap welding machine which is arranged on a process line to continuously produce a steel plate and which has a welding part 43 to lap the rear end part of an advancing plate and the top end part of a following plate and and to weld them, and planishing rolls 71, 74 to roll the deposited part 43 of the steel plate welded with this welding part 43, the planishing rolls 71, 74 are arranged so as to be crossed by a right angle to the transferring direction of the steel plate and the deposited part 43 is rolled to the transferring direction of the steel plate.