A crimp-binding type of sheet binding device comprising: a pair of toothing crimping members (261a, 261b); and a movable-crimping-member moving unit that moves a movable crimping member which is one of the pair of crimping members (261a, 261b) and is movably installed, wherein the sheet binding device binds a sheet bundle by causing the movable-crimping-member moving unit to move the movable crimping member thereby holding the sheet bundle between the pair of crimping members (261a, 261b), and the movable-crimping-member moving unit includes: a link mechanism that includes a first link member having one end rotatably connected to the movable crimping member, a second link member having one end rotatably connected to a fixed member fixed to a device body, and a connecting part that rotatably connects the other end of the first link member and the other end of the second link member; and being characterised by a connecting member (271 a) having one end rotatably connected to the connecting part of the link mechanism, and capable of moving between a first position causing the link mechanism to be extended and a second position retracted from the first position and causing the link mechanism to be more flexed than in the first position; and a connecting-member moving unit (271, 277) that includes a rotary member (271c, 276) capable of rotating on a displaceable rotating shaft (271b, 276a), and reciprocates the connecting member (271 a) between the first position and the second position by rotation of the rotary member (271c, 276) in one direction. a pair of toothing crimping members (261a, 261b); and a movable-crimping-member moving unit that moves a movable crimping member which is one of the pair of crimping members (261a, 261b) and is movably installed, wherein the sheet binding device binds a sheet bundle by causing the movable-crimping-member moving unit to move the movable crimping member thereby holding the sheet bundle between the pair of crimping members (261a, 261b), and the movable-crimping-member moving unit includes: a link mechanism that includes a first link member having one end rotatably connected to the movable crimping member, a second link member having one end rotatably connected to a fixed member fixed to a device body, and a connecting part that rotatably connects the other end of the first link member and the other end of the second link member; and being characterised by a connecting member (271 a) having one end rotatably connected to the connecting part of the link mechanism, and capable of moving between a first position causing the link mechanism to be extended and a second position retracted from the first position and causing the link mechanism to be more flexed than in the first position; and a connecting-member moving unit (271, 277) that includes a rotary member (271c, 276) capable of rotating on a displaceable rotating shaft (271b, 276a), and reciprocates the connecting member (271 a) between the first position and the second position by rotation of the rotary member (271c, 276) in one direction. a link mechanism that includes a first link member having one end rotatably connected to the movable crimping member, a second link member having one end rotatably connected to a fixed member fixed to a device body, and a connecting part that rotatably connects the other end of the first link member and the other end of the second link member; and being characterised by a connecting member (271 a) having one end rotatably connected to the connecting part of the link mechanism, and capable of moving between a first position causing the link mechanism to be extended and a second position retracted from the first position and causing the link mechanism to be more flexed than in the first position; and a connecting-member moving unit (271, 277) that includes a rotary member (271c, 276) capable of rotating on a displaceable rotating shaft (271b, 276a), and reciprocates the connecting member (271 a) between the first position and the second position by rotation of the rotary member (271c, 276) in one direction.