A shock absorber comprising: a cylinder sealingly containing hydraulic fluid therein; a piston inserted in the cylinder and dividing an inside of the cylinder into a rod-side fluid chamber and an opposite rod-side fluid chamber; a piston rod coupled with the piston and extending out of the cylinder; a flow passage (11, 12) configured in such a manner that a flow of the hydraulic fluid is generated due to extension and compression of the piston rod; and a valve mechanism (21, 31) provided in the flow passage (11, 12) and configured to generate a damping force, wherein the valve mechanism (21, 31) includes an annular valve seat member fixably provided by being inserted at an intermediate position of the flow passage (11, 12), the valve seat member having one end side serving as a valve seat, the valve seat member including a through-hole at a central portion thereof, a valve pin including a large-diameter portion and a small-diameter portion, the large-diameter portion forming a valve body by abutting against the valve seat, the small-diameter portion having one side connected to the large-diameter portion and the other side inserted on an inner peripheral side of the valve seat member, a passage formed between the through-hole of the valve seat member and an outer periphery of the valve pin and configured to be opened/closed according to a movement of the valve pin, characterised in that a nut is threadably engaged with an outer periphery of the other side of the small-diameter portion of the valve pin, and an elastic member is disposed between the other end side of the valve seat member and the nut, wherein the flow passage (11, 12) is provided at the piston, and at least a part of the nut is disposed in the flow passage (11, 12). a cylinder sealingly containing hydraulic fluid therein; a piston inserted in the cylinder and dividing an inside of the cylinder into a rod-side fluid chamber and an opposite rod-side fluid chamber; a piston rod coupled with the piston and extending out of the cylinder; a flow passage (11, 12) configured in such a manner that a flow of the hydraulic fluid is generated due to extension and compression of the piston rod; and a valve mechanism (21, 31) provided in the flow passage (11, 12) and configured to generate a damping force, wherein the valve mechanism (21, 31) includes an annular valve seat member fixably provided by being inserted at an intermediate position of the flow passage (11, 12), the valve seat member having one end side serving as a valve seat, the valve seat member including a through-hole at a central portion thereof, a valve pin including a large-diameter portion and a small-diameter portion, the large-diameter portion forming a valve body by abutting against the valve seat, the small-diameter portion having one side connected to the large-diameter portion and the other side inserted on an inner peripheral side of the valve seat member, a passage formed between the through-hole of the valve seat member and an outer periphery of the valve pin and configured to be opened/closed according to a movement of the valve pin, characterised in that a nut is threadably engaged with an outer periphery of the other side of the small-diameter portion of the valve pin, and an elastic member is disposed between the other end side of the valve seat member and the nut,