A gas supply system includes a gas tank with a self-closing valve, a gas supply pipe that connects the gas tank and a gas consuming device, a sealing, and a check valve disposed in the gas supply pipe. A push rod is provided to the gas supply pipe, and pushes open the self-closing valve. The sealing seals off a connection space including the self-closing valve and the distal end of the gas supply pipe when a distance between the self-closing valve and the push rod is equal to or less than a threshold value distance. A controller moves the gas tank forward until the self-closing valve is pushed open, and then backward to a position where the self-closing valve closes while maintaining sealing off of the connection space. The controller determines that a gas leak is occurring when pressure upstream of the check valve is lower than pressure downstream.