The invention relates to a combustion engine which is provided with an annular cylinder, an output shaft concentrically arranged with the cylinder, and two rotating bodies concentrically arranged with the cylinder, wherein n pistons are arranged on each rotating body, the pistons on the two rotating bodies divide the cylinder into 2n working chambers with variable volumes, the pistons move in the cylinder through the pushing of combusting gas, the pistons drive two push rods through sleeves to synchronously step and rotate with the rotating bodies when pushing the rotating bodies to step and rotate, the push rods push a rotor through a sliding block to revolute and rotate around the output shaft, the rotor pushes the output shaft through a gear mechanism to continuously rotate, the output shaft rotates for n-1 circles, and the rotor reversely rotates n circles. The combustion engine has the advantages of simple structure, easy manufacture, low cost, better fuel oil economical efficiency and the like.