To provide an oil tank device for an accumulated pressure braking energy regenerating apparatus capable of preventing the suction of a sealed inert gas from and operation oil inlet/outlet.

An inside opening open to the inside of an oil tank at an operating oil in/outlet provided in the oil tank is located corresponding to the oil tank on the front side of a vehicle and on the lower side. Specifically, the oil tank T is constructed by one container 1 extending to the longitudinal direction along the front and rear side of the vehicle and having a pipe portion 2 constituting the opening oil access, so that the inside opening 2A of the pipe portion 2 can be located at the lower side of the front part of the container 1.