In an internal combustion engine provided with a decompression mechanism, a decompression cam that is rotatable with respect to a camshaft between first and second stop positions has a cam profile so that an exhaust valve is opened at the first stop position and is closed at the second stop position. The decompression cam is rotated in the reverse direction to the first stop position by rotating a crankshaft in the reverse direction by an electric motor at startup (position P1). Only the crankshaft is then rotated in the reverse direction (position p3), and the decompression cam is rotated in the normal direction by rotating the crankshaft in the normal direction by the electric motor. During either a compression strokes included within a range of a reverse rotation angle or the first compression stroke after initiation of normal rotation of the decompression cam until the decompression cam reaches the second stop position, the decompression cam opens the exhaust valve and increases the run-up angle of the crankshaft. The aforementioned method and device facilitates a piston overcoming the first compression top dead center decompression operation has stopped without unduly increasing the size and capacity of the required electric motor.