In a marine gas cylinder apparatus provided with a switching valve apparatus capable of switching a communication state among a piston rod side oil chamber, a piston side oil chamber and a gas chamber, the switching valve apparatus is structured such that each of respective poppet valves are arranged in each of ports open to a communication chamber provided in a valve case, each of balls is held between a ball operation surface of a rotation plate supported to the valve case and each of the poppet valves. Each of the poppet valves is moved close to or apart from the corresponding port via each of the balls in accordance with a rotating operation of the rotation plate.