A shock absorber (D1, D2), comprising: a cylinder; a piston slidably inserted into the cylinder, the piston defining an expansion side chamber and a pressure side chamber inside the cylinder; a reservoir; a suction passage that allows only a flow of liquid from the reservoir toward the pressure side chamber; a rectifying passage that allows only a flow of liquid from the pressure side chamber toward the expansion side chamber; and a damping force adjusting unit that allows only a flow of liquid from the expansion side chamber toward the reservoir, the damping force adjusting unit being configured to change resistance provided to the flow of liquid, characterized by further comprising: a housing that forms a pressure chamber; and a free piston slidably inserted into the pressure chamber to form an expansion-side pressure chamber and a pressure-side pressure chamber inside the pressure chamber, the expansion-side pressure chamber communicating with the expansion side chamber, the pressure-side pressure chamber communicating with the pressure side chamber, wherein a pressure derived from the pressure side chamber acts on the free piston to press the free piston to one side in a sliding direction, and a pressure derived from the expansion side chamber acts on the free piston to press the free piston to another side in the sliding direction, a pressure-side pressure-receiving area on which the pressure-side-chamber-derived pressure of the free piston acts is larger than an expansion-side pressure-receiving area on which the expansion-side-chamber-derived pressure of the free piston acts, and the shock absorber (D1, D2) further comprises: an outer pipe disposed outside the cylinder; and an intermediate pipe disposed between the cylinder and the outer pipe, wherein the housing fits end portions of the cylinder and the intermediate pipe, the reservoir is formed between the intermediate pipe and the outer pipe, a gap between the cylinder and the intermediate pipe forms a discharge passage, the discharge passage causing the expansion side chamber to communicate with the reservoir, the damping force adjusting unit is disposed between the discharge passage and the reservoir, and the expansion-side pressure chamber communicates with the expansion side chamber via the discharge passage. a cylinder; a piston slidably inserted into the cylinder, the piston defining an expansion side chamber and a pressure side chamber inside the cylinder; a reservoir; a suction passage that allows only a flow of liquid from the reservoir toward the pressure side chamber; a rectifying passage that allows only a flow of liquid from the pressure side chamber toward the expansion side chamber; and a damping force adjusting unit that allows only a flow of liquid from the expansion side chamber toward the reservoir, the damping force adjusting unit being configured to change resistance provided to the flow of liquid, characterized by further comprising: a housing that forms a pressure chamber; and a free piston slidably inserted into the pressure chamber to form an expansion-side pressure chamber and a pressure-side pressure chamber inside the pressure chamber, the expansion-side pressure chamber communicating with the expansion side chamber, the pressure-side pressure chamber communicating with the pressure side chamber, wherein a housing that forms a pressure chamber; and a free piston slidably inserted into the pressure chamber to form an expansion-side pressure chamber and a pressure-side pressure chamber inside the pressure chamber, the expansion-side pressure chamber communicating with the expansion side chamber, the pressure-side pressure chamber communicating with the pressure side chamber, wherein a pressure derived from the pressure side chamber acts on the free piston to press the free piston to one side in a sliding direction, and a pressure derived from the expansion side chamber acts on the free piston to press the free piston to another side in the sliding direction, a pressure-side pressure-receiving area on which the pressure-side-chamber-derived pressure of the free piston acts is larger than an expansion-side pressure-receiving area on which the expansion-side-chamber-derived pressure of the free piston acts, and the shock absorber (D1, D2) further comprises: an outer pipe disposed outside the cylinder; and an intermediate pipe disposed between the cylinder and the outer pipe, wherein the housing fits end portions of the cylinder and the intermediate pipe, the reservoir is formed between the intermediate pipe and the outer pipe, a gap between the cylinder and the intermediate pipe forms a discharge passage, the discharge passage causing the expansion side chamber to communicate with the reservoir, the damping force adjusting unit is disposed between the discharge passage and the reservoir, and the expansion-side pressure chamber communicates with the expansion side chamber via the discharge passage. an outer pipe disposed outside the cylinder; and an intermediate pipe disposed between the cylinder and the outer pipe, wherein the housing fits end portions of the cylinder and the intermediate pipe, the reservoir is formed between the intermediate pipe and the outer pipe, a gap between the cylinder and the intermediate pipe forms a discharge passage, the discharge passage causing the expansion side chamber to communicate with the reservoir, the damping force adjusting unit is disposed between the discharge passage and the reservoir, and the expansion-side pressure chamber communicates with the expansion side chamber via the discharge passage.