A seatbelt retractor comprising: a housing; a take-up drum that is housed in the housing in a rotatable fashion and configured to house a webbing by winding the webbing thereon; and a pretensioner mechanism unit that is mounted on an exterior of one-side wall portion of the housing and configured to rotate the take-up drum in a webbing-take-up direction so as to take up the webbing on the take-up drum in a case of vehicle collision, an axial directional one end of the take-up drum is rotatably supported by the one-side wall portion of the housing and a shaft portion erected on an axial directional other end of the take-up drum is rotatably supported by a cover member that is mounted on an exterior of the other-side wall portion of the housing, the other-side wall portion facing the one-side wall portion, and the housing includes: an opening portion that is formed in the other-side wall portion of the housing and configured to allow insertion of a ratchet gear formed on an outer periphery of an edge portion at the axial directional other end of the take-up drum so that predetermined space is secured between the opening portion and the ratchet gear when the ratchet gear is inserted therein; and inner teeth that are formed on an inner periphery of the opening portion so as to face the ratchet gear and capable of engaging with the ratchet gear, said inner teeth being formed so as to tilt in a direction where the take-up drum rotates when taking up the webbing, and when the pretensioner mechanism unit activates and causes the webbing to receive a predetermined value or more of tensile force, the cover member gets plastically deformed via the shaft portion so that the inner teeth of the opening portion and the ratchet gear stay in an engaged state, characterized in that pitch of the inner teeth is smaller than pitch of teeth of the ratchet gear. a housing; a take-up drum that is housed in the housing in a rotatable fashion and configured to house a webbing by winding the webbing thereon; and a pretensioner mechanism unit that is mounted on an exterior of one-side wall portion of the housing and configured to rotate the take-up drum in a webbing-take-up direction so as to take up the webbing on the take-up drum in a case of vehicle collision, an axial directional one end of the take-up drum is rotatably supported by the one-side wall portion of the housing and a shaft portion erected on an axial directional other end of the take-up drum is rotatably supported by a cover member that is mounted on an exterior of the other-side wall portion of the housing, the other-side wall portion facing the one-side wall portion, and the housing includes: an opening portion that is formed in the other-side wall portion of the housing and configured to allow insertion of a ratchet gear formed on an outer periphery of an edge portion at the axial directional other end of the take-up drum so that predetermined space is secured between the opening portion and the ratchet gear when the ratchet gear is inserted therein; and inner teeth that are formed on an inner periphery of the opening portion so as to face the ratchet gear and capable of engaging with the ratchet gear, said inner teeth being formed so as to tilt in a direction where the take-up drum rotates when taking up the webbing, and an opening portion that is formed in the other-side wall portion of the housing and configured to allow insertion of a ratchet gear formed on an outer periphery of an edge portion at the axial directional other end of the take-up drum so that predetermined space is secured between the opening portion and the ratchet gear when the ratchet gear is inserted therein; and inner teeth that are formed on an inner periphery of the opening portion so as to face the ratchet gear and capable of engaging with the ratchet gear, said inner teeth being formed so as to tilt in a direction where the take-up drum rotates when taking up the webbing, and when the pretensioner mechanism unit activates and causes the webbing to receive a predetermined value or more of tensile force, the cover member gets plastically deformed via the shaft portion so that the inner teeth of the opening portion and the ratchet gear stay in an engaged state, characterized in that pitch of the inner teeth is smaller than pitch of teeth of the ratchet gear.