A magnetorheological fluid shock absorber comprising: a cylinder in which a magnetorheological fluid whose viscosity is changed by an action of a magnetic field is sealed; a piston slidably arranged in the cylinder and defining a pair of fluid chambers in the cylinder; and a piston rod connected to the piston and extending to an outside of the cylinder, wherein the piston includes: a piston core having a small diameter portion mounted on an end portion of the piston rod, an enlarged diameter portion formed continuously in an axial direction with a diameter larger than the small diameter portion and forming a stepped portion between the enlarged diameter portion and the small diameter portion, and a large diameter portion formed continuously in the axial direction with a diameter larger than the enlarged diameter portion and having a coil provided on an outer periphery; a ring body surrounding the outer periphery of the piston core and forming a flow passage of the magnetorheological fluid between the ring body and the large diameter portion characterised in that a plate formed annularly and arranged on the outer periphery of the small diameter portion and mounted on one end of the ring body; and a stopper mounted on the small diameter portion and sandwiching the plate between the stopper and the stepped portion. a cylinder in which a magnetorheological fluid whose viscosity is changed by an action of a magnetic field is sealed; a piston slidably arranged in the cylinder and defining a pair of fluid chambers in the cylinder; and a piston rod connected to the piston and extending to an outside of the cylinder, wherein the piston includes: a piston core having a small diameter portion mounted on an end portion of the piston rod, an enlarged diameter portion formed continuously in an axial direction with a diameter larger than the small diameter portion and forming a stepped portion between the enlarged diameter portion and the small diameter portion, and a large diameter portion formed continuously in the axial direction with a diameter larger than the enlarged diameter portion and having a coil provided on an outer periphery; a ring body surrounding the outer periphery of the piston core and forming a flow passage of the magnetorheological fluid between the ring body and the large diameter portion characterised in that a plate formed annularly and arranged on the outer periphery of the small diameter portion and mounted on one end of the ring body; and a stopper mounted on the small diameter portion and sandwiching the plate between the stopper and the stepped portion. a piston core having a small diameter portion mounted on an end portion of the piston rod, an enlarged diameter portion formed continuously in an axial direction with a diameter larger than the small diameter portion and forming a stepped portion between the enlarged diameter portion and the small diameter portion, and a large diameter portion formed continuously in the axial direction with a diameter larger than the enlarged diameter portion and having a coil provided on an outer periphery; a ring body surrounding the outer periphery of the piston core and forming a flow passage of the magnetorheological fluid between the ring body and the large diameter portion characterised in that a plate formed annularly and arranged on the outer periphery of the small diameter portion and mounted on one end of the ring body; and a stopper mounted on the small diameter portion and sandwiching the plate between the stopper and the stepped portion.