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 (B) and an opposite rod-side fluid chamber (C); a piston rod coupled with the piston and extending out of the cylinder ; a flow passage 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 provided in the flow passage and configured to generate a damping force, wherein the valve mechanism includes an annular valve seat member fixably provided by being inserted at an intermediate position of the flow passage , 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 is provided at the piston , and at least a part of the nut is disposed in the flow passage . 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 (B) and an opposite rod-side fluid chamber (C); a piston rod coupled with the piston and extending out of the cylinder ; a flow passage 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 provided in the flow passage and configured to generate a damping force, wherein the valve mechanism includes an annular valve seat member fixably provided by being inserted at an intermediate position of the flow passage , 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 , <EOS>