A rotary damper, comprising: a housing including an outer cylinder having a predetermined axis line, an inner cylinder coaxially disposed inside the outer cylinder, and a bottom plate closing one end side in an axis line direction of the outer cylinder and the inner cylinder, wherein an annular liquid chamber is defined by the outer cylinder, the inner cylinder, and the bottom plate, and a viscous fluid is filled in the liquid chamber; a rotor including a cylindrical blade portion rotatably received in the liquid chamber coaxially with the outer cylinder; an annular locking groove extending in a circumferential direction in one of either a first portion which is an outer peripheral surface of the inner cylinder and is located at an end edge portion on a side opposed to a bottom plate side, or a second portion facing the first portion on an inner peripheral surface of the blade portion; a locking convex portion projecting from the other of either the first portion or the second portion, and locked in the locking groove so as to control a relative movement in the axis line direction between the housing and the rotor; an annular inside seal member interposed between a portion on the outer peripheral surface of the inner cylinder at an upper side of a step portion of the inner cylinder, and the inner peripheral surface of the blade portion; and an annular outside seal member interposed between a vicinity of the other end in the axis line direction on an inner peripheral surface of the outer cylinder, and an outer peripheral surface of the blade portion, said rotary damper being characterized in that in a small diameter portion, there is formed a pair of slits extending downward from an upper end edge thereof; the slits are formed at symmetric positions around the axis line, and extend up to an intermediate portion in the up-and-down direction of the small diameter portion by forming the pair of slits, and upper half portion of the small diameter portion is divided into two arc-like pieces, and can tilt slightly inward in a radial direction, and on the first portion of an upper half portion of the small diameter portion, a locking groove extending in a circumferential direction is formed, and the locking groove is continuous in the circumferential direction through the pair of slits and has an angular shape. a housing including an outer cylinder having a predetermined axis line, an inner cylinder coaxially disposed inside the outer cylinder, and a bottom plate closing one end side in an axis line direction of the outer cylinder and the inner cylinder, wherein an annular liquid chamber is defined by the outer cylinder, the inner cylinder, and the bottom plate, and a viscous fluid is filled in the liquid chamber; a rotor including a cylindrical blade portion rotatably received in the liquid chamber coaxially with the outer cylinder; an annular locking groove extending in a circumferential direction in one of either a first portion which is an outer peripheral surface of the inner cylinder and is located at an end edge portion on a side opposed to a bottom plate side, or a second portion facing the first portion on an inner peripheral surface of the blade portion; a locking convex portion projecting from the other of either the first portion or the second portion, and locked in the locking groove so as to control a relative movement in the axis line direction between the housing and the rotor; an annular inside seal member interposed between a portion on the outer peripheral surface of the inner cylinder at an upper side of a step portion of the inner cylinder, and the inner peripheral surface of the blade portion; and an annular outside seal member interposed between a vicinity of the other end in the axis line direction on an inner peripheral surface of the outer cylinder, and an outer peripheral surface of the blade portion, said rotary damper being characterized in that in a small diameter portion, there is formed a pair of slits extending downward from an upper end edge thereof; the slits are formed at symmetric positions around the axis line, and extend up to an intermediate portion in the up-and-down direction of the small diameter portion by forming the pair of slits, and upper half portion of the small diameter portion is divided into two arc-like pieces, and can tilt slightly inward in a radial direction, and on the first portion of an upper half portion of the small diameter portion, a locking groove extending in a circumferential direction is formed, and the locking groove is continuous in the circumferential direction through the pair of slits and has an angular shape.