Hydraulic fluid in a cylinder's lower chamber is introduced into a back pressure chamber through the outer periphery of an extension-side main valve during a compression stroke of a piston rod. This eliminates the necessity of forming paths by arranging a plurality of discs and a check valve, the paths for introducing into the back pressure chamber the hydraulic fluid in the cylinder's lower chamber that becomes an upstream chamber during the compression stroke. This makes it possible to decrease the axial length of a shaft portion of the piston rod and therefore the entire length of a cylinder, and also downsize a shock absorber.