A decompression device is attached to an exhaust camshaft, and the exhaust camshaft is supported by a cylinder head. The decompression device includes a decompression camshaft formed with a decompression cam that can protrude and be immersed with respect to a base circle of an exhaust cam of the exhaust camshaft, a decompression arm that moves in an opening direction due to centrifugal force accompanying rotation of the exhaust camshaft to protrude the decompression cam, and a spring that moves the decompression arm in a closing direction by spring force resisting the centrifugal force to immerse the decompression cam. In the decompression device, the opening direction of the decompression arm can be changed in a same direction or in an opposite direction to a rotation direction of the exhaust camshaft for each cylinder.