PROBLEM TO BE SOLVED: To effectively function a balance weight by contriving the shape and mounting of the balance weight.SOLUTION: A walk-behind working machine includes a mounting object part 9C to which a mounting base 12A provided in a balance weight 12 is demountably mounted, at a front lower side of a machine body frame 1. In a balance weight mounted state where the mounting base 12A is mounted on the mounting object part 9C, an extension part 12B of the balance weight 12 is positioned at a machine body front side with respect to a front end of the machine frame 1.