A decompression apparatus includes a plunger and a decompression shaft. The plunger is provided in a plunger hole formed in a cam shaft which is configured to move an engine valve of an internal combustion engine. The plunger is movable in the plunger hole between a decompression position in which the plunger opens the engine valve and a decompression cancel position in which the plunger does not open the engine valve. The plunger hole has a first opening which partially locates on a cam surface of a valve cam which is provided on the cam shaft. A first end portion of the plunger protrudes from the first opening to open the engine valve in the decompression position. The decompression shaft is provided in the cam shaft and configured to move the plunger between the decompression position and the decompression cancel position.