An applicator for fitting a resilient ring device (21, 22) around a rod device (20, 20a) by radial expansion, comprising: a movable sleeve (14, 61, 84) for receiving entry of said rod device; support means (11, 13, 62, 69, 83, 92) for supporting said movable sleeve movably; at least three arm devices (12, 46, 47, 63, 86), having a rear end portion, arranged around said movable sleeve circumferentially, for extending in said axial direction, said rear end portion being secured to said support means in a radial direction of said movable sleeve; a receiving portion (12c, 46c, 63c, 86c), formed with a front end portion of said arm devices, for receiving an inner edge of said ring device, and moving to open and closed state positions upon pivotal movement of said arm devices, said receiving portion, when in said closed state position, being adapted to setting said ring device thereon, and when in said open state position, being adapted to set said ring device around said rod device; a shift mechanism (15, 50, 70, 82, 94) for shifting said movable sleeve relative to said support means, to move said arm devices by engaging said movable sleeve with said arm devices, to move said receiving portion from said closed state position to said open state position, characterized in that the support means supports the movable sleeve movably in an axial direction (A3, A4); that the rear end portion is secured to the support means in a pivotally movable manner, that the shift mechanism is adapted to shift the movable sleeve so as to move the arm devices pivotally and to advance said ring device from said receiving portion with an end surface of said movable sleeve (14, 61, 84). a movable sleeve (14, 61, 84) for receiving entry of said rod device; support means (11, 13, 62, 69, 83, 92) for supporting said movable sleeve movably; at least three arm devices (12, 46, 47, 63, 86), having a rear end portion, arranged around said movable sleeve circumferentially, for extending in said axial direction, said rear end portion being secured to said support means in a radial direction of said movable sleeve; a receiving portion (12c, 46c, 63c, 86c), formed with a front end portion of said arm devices, for receiving an inner edge of said ring device, and moving to open and closed state positions upon pivotal movement of said arm devices, said receiving portion, when in said closed state position, being adapted to setting said ring device thereon, and when in said open state position, being adapted to set said ring device around said rod device; a shift mechanism (15, 50, 70, 82, 94) for shifting said movable sleeve relative to said support means, to move said arm devices by engaging said movable sleeve with said arm devices, to move said receiving portion from said closed state position to said open state position, characterized in that the support means supports the movable sleeve movably in an axial direction (A3, A4); that the rear end portion is secured to the support means in a pivotally movable manner, that the shift mechanism is adapted to shift the movable sleeve so as to move the arm devices pivotally and to advance said ring device from said receiving portion with an end surface of said movable sleeve (14, 61, 84).