An apparatus for compressing/expanding a working gas (compression/expansion apparatus), including: a cylinder; a piston for compressing/expanding said working gas in a working space (front space) of said cylinder; a crank mechanism connected with a piston rod of said piston via an oil seal; a crank chamber for housing therein said crank mechanism and communicating with said cylinder via said oil seal, said apparatus comprising: a first tube allowing for a unidirectional flow of the working gas from said crank chamber to said front space in said cylinder, said first tube having therein an oil filter and a first check valve; and a second tube allowing for a unidirectional flow of the working gas from said front space to a second space (rear space) behind said piston in said cylinder, said second tube having therein a second check valve. In this construction a loop of the working gas is established for pumping the gas from the crank chamber to the rear space of the cylinder via the front space, and then back to the crank chamber via the oil seal between the cylinder and the crank chamber. The passage of the gas through the oil seal helps prevent the lubrication oil from infiltrating directly from the crank chamber into the cylinder along the piston rod.