A magnetorheological fluid shock absorber includes a piston slidably arranged in a cylinder in which magnetorheological fluid whose viscosity changes by the action of a magnetic field is sealed. The piston includes a piston core which is attached to an end part of the piston rod and on the outer periphery of which a coil is provided, a flux ring which surrounds the outer periphery of the piston core and forms a flow passage for the magnetorheological fluid between the piston core and the flux ring, a plate which is annularly formed, arranged on the outer periphery of the piston rod and attached to one end of the flux ring, and a stopper whose axial position is specified with respect to the piston rod and which sandwiches the plate between the piston core and the stopper.