A shock absorber, comprising: a cylinder; a piston slidably inserted into the cylinder, the piston defining an expansion side chamber and an 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; a housing that includes a pressure chamber including a small-cross-sectional-area portion and a large-cross-sectional-area portion; a free piston that includes: a small-piston portion slidably inserted into the small-cross-sectional-area portion of the pressure chamber; and a large-piston portion slidably inserted into the large-cross-sectional-area portion of the pressure chamber, the free piston defining a small chamber inside the small-cross-sectional-area portion using the small-piston portion, the free piston defining an outer peripheral chamber in an outer periphery of the small-piston portion inside the large-cross-sectional-area portion, the free piston defining a large chamber inside the large-cross-sectional-area portion using the large-piston portion; a spring element configured to position the free piston in a center position of the pressure chamber, the spring element being configured to generate a biasing force to reduce displacement of the free piston from the center position; a pressure-side first passage and a pressure-side second passage that communicate between the large chamber and the pressure side chamber; a first valve (Va, 32, 44) disposed in the pressure-side first passage, the first valve (Va, 32, 44) being configured to allow only a flow of liquid from the large chamber toward the pressure side chamber while providing resistance to the flow; and a second valve (Vb, 33, 45) disposed in the pressure-side second passage, the second valve (Vb, 33, 45) being configured to allow only a flow of liquid from the pressure side chamber toward the large chamber while providing resistance of the flow, characterized by an expansion side passage that causes one of the small chamber and the outer peripheral chamber to communicate with the expansion side chamber. a cylinder; a piston slidably inserted into the cylinder, the piston defining an expansion side chamber and an 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; a housing that includes a pressure chamber including a small-cross-sectional-area portion and a large-cross-sectional-area portion; a free piston that includes: a small-piston portion slidably inserted into the small-cross-sectional-area portion of the pressure chamber; and a large-piston portion slidably inserted into the large-cross-sectional-area portion of the pressure chamber, the free piston defining a small chamber inside the small-cross-sectional-area portion using the small-piston portion, the free piston defining an outer peripheral chamber in an outer periphery of the small-piston portion inside the large-cross-sectional-area portion, the free piston defining a large chamber inside the large-cross-sectional-area portion using the large-piston portion; a spring element configured to position the free piston in a center position of the pressure chamber, the spring element being configured to generate a biasing force to reduce displacement of the free piston from the center position; a pressure-side first passage and a pressure-side second passage that communicate between the large chamber and the pressure side chamber; a first valve (Va, 32, 44) disposed in the pressure-side first passage, the first valve (Va, 32, 44) being configured to allow only a flow of liquid from the large chamber toward the pressure side chamber while providing resistance to the flow; and a second valve (Vb, 33, 45) disposed in the pressure-side second passage, the second valve (Vb, 33, 45) being configured to allow only a flow of liquid from the pressure side chamber toward the large chamber while providing resistance of the flow, characterized by an expansion side passage that causes one of the small chamber and the outer peripheral chamber to communicate with the expansion side chamber.