A decompression mechanism provided in a valve operating system configured to drive a valve for opening and closing a port, includes an accommodating member provided in the valve operating system; a decompression member which is accommodated in the accommodating member such that the decompression member is extendable and retractable, the decompression member being configured to extend from the valve operating system to press the valve to open the port in a compression stroke of the engine; and an insertion member which is inserted into the valve operating system, the insertion member being configured to be inserted through the accommodating member in a direction crossing a direction in which the decompression member is extendable and retractable.