To provide a method for improving the fatigue strength of a lap welded metal joint by which the fatigue strength of the lap welded metal joint can be stably improved irrespective of the material of a metal plate and the type of objective members.

Firstly, a center portion 10a of a weld metal 2 of a lap joint where two metal plates 1a, 1b has been lap-welded is subjected to ultrasonic shock treatment. Thereafter, an immediately above area 10b of a boundary 4 between a space 3 formed in a lapped portion (the lap welded joint part) of the metal plate 1a and the metal plate 1b and the weld metal 2 is subjected to the ultrasonic shock treatment with the width of t-2t mm, where t is the thickness of the metal plate to be treated.