A shock absorber (D1-D5) comprising: a cylinder which is filled with a fluid; a piston that is slidably inserted into the cylinder and partitions the inside of the cylinder into an extension-side chamber and a compression-side chamber; a piston rod that is connected at one end to the piston and projects at the other end toward the outside of the cylinder; a reservoir configured to compensate for a change in capacity in the cylinder accompanying an ingression and retraction of the piston rod into and out of the cylinder; a suction passage configured to permit only a flow of the fluid from the reservoir toward the compression-side chamber; a rectification passage configured to permit only a flow of the fluid from the compression-side chamber toward the extension-side chamber; a damping force adjusting part configured to permit only a flow of the fluid from the extension-side chamber toward the reservoir and be capable of changing a resistance applied to the flow of the fluid; a housing that has a pressure chamber therein; a free piston that is slidably inserted into the pressure chamber and forms an extension-side pressure chamber and a compression-side pressure chamber within the pressure chamber; and a spring element configured to position the free piston in a neutral position within the pressure chamber and exert a biasing force to suppress displacement of the free piston from the neutral position, wherein the extension-side chamber communicates with the extension-side pressure chamber so as to press the free piston in one sliding direction, and the compression-side chamber communicates with the compression-side pressure chamber so as to press the free piston in the other sliding direction, characterized in that a pressure-receiving area of the free piston upon which a pressure of the compression-side pressure chamber acts is larger than a pressure-receiving area of the free piston upon which a pressure of the extension-side pressure chamber acts, and the shock absorber (D1-D5) comprises: an outer tube provided on the outside of the cylinder; an intermediate tube provided between the cylinder and the outer tube; and a discharge passage that is formed between the cylinder and the intermediate tube and causes the extension-side chamber and the reservoir to communicate with each other, wherein the housing is fitted to an end of the cylinder and the intermediate tube, the reservoir is formed between the intermediate tube and the outer tube, the damping force adjusting part is provided between the discharge passage and the reservoir, and the extension-side chamber communicates with the extension-side pressure chamber through the discharge passage. a cylinder which is filled with a fluid; a piston that is slidably inserted into the cylinder and partitions the inside of the cylinder into an extension-side chamber and a compression-side chamber; a piston rod that is connected at one end to the piston and projects at the other end toward the outside of the cylinder; a reservoir configured to compensate for a change in capacity in the cylinder accompanying an ingression and retraction of the piston rod into and out of the cylinder; a suction passage configured to permit only a flow of the fluid from the reservoir toward the compression-side chamber; a rectification passage configured to permit only a flow of the fluid from the compression-side chamber toward the extension-side chamber; a damping force adjusting part configured to permit only a flow of the fluid from the extension-side chamber toward the reservoir and be capable of changing a resistance applied to the flow of the fluid; a housing that has a pressure chamber therein; a free piston that is slidably inserted into the pressure chamber and forms an extension-side pressure chamber and a compression-side pressure chamber within the pressure chamber; and a spring element configured to position the free piston in a neutral position within the pressure chamber and exert a biasing force to suppress displacement of the free piston from the neutral position, wherein the extension-side chamber communicates with the extension-side pressure chamber so as to press the free piston in one sliding direction, and the compression-side chamber communicates with the compression-side pressure chamber so as to press the free piston in the other sliding direction, characterized in that a pressure-receiving area of the free piston upon which a pressure of the compression-side pressure chamber acts is larger than a pressure-receiving area of the free piston upon which a pressure of the extension-side pressure chamber acts, and the shock absorber (D1-D5) comprises: an outer tube provided on the outside of the cylinder; an intermediate tube provided between the cylinder and the outer tube; and a discharge passage that is formed between the cylinder and the intermediate tube and causes the extension-side chamber and the reservoir to communicate with each other, an outer tube provided on the outside of the cylinder; an intermediate tube provided between the cylinder and the outer tube; and a discharge passage that is formed between the cylinder and the intermediate tube and causes the extension-side chamber and the reservoir to communicate with each other, wherein the housing is fitted to an end of the cylinder and the intermediate tube, the reservoir is formed between the intermediate tube and the outer tube, the damping force adjusting part is provided between the discharge passage and the reservoir, and the extension-side chamber communicates with the extension-side pressure chamber through the discharge passage.