A working machine engine is provided. The air flowing through the front side cooling air flow passage and the upper side cooling air flow passage which are provided in the front side and the upper side of the casing, respectively, and the air flowing through the auxiliary air flow passage provided in the back side of the casing join together near the opening. By this means, the air flowing through the auxiliary air flow passage can change the direction of the air flowing lengthwise through the front side cooling air flow passage and the upper side cooling air flow passage.
