A trimmer that trims a bound material, comprising: a pusher that pushes the bound material; a pusher moving device that moves the pusher; a positioning part that positions the bound material pushed by the pusher; characterised by a position detector that detects a stopped position where the pusher is stopped; a reaction force detector that detects a reaction force to which the pusher is subject; a set position memory that memorizes a set standby position and a set stop position, the set standby position being a position where the pusher should be standby, the set stop position being a position where the pusher should be stopped; an allowable position memory that memorizes an allowable position range relevant to the pusher; an allowable reaction force memory that memorizes an allowable reaction force range relevant to the pusher; and a determiner that determines whether a length of the bound material is normal or abnormal; wherein the pusher moving device is configured to stop the pusher when the reaction force of the pusher arrives at an upper limit of the allowable reaction force range while the pusher is moved from the set standby position toward the set stop position; and wherein the determiner determines that: the length of the bound material is normal when the stopped position of the pusher is in the allowable position range, and the reaction force of the pusher is in the allowable reaction force range; or the length of the bound material is abnormal when the stopped position of the pusher is out of the allowable position range, or the reaction force of the pusher is out of the allowable reaction force range. a pusher that pushes the bound material; a pusher moving device that moves the pusher; a positioning part that positions the bound material pushed by the pusher; characterised by a position detector that detects a stopped position where the pusher is stopped; a reaction force detector that detects a reaction force to which the pusher is subject; a set position memory that memorizes a set standby position and a set stop position, the set standby position being a position where the pusher should be standby, the set stop position being a position where the pusher should be stopped; an allowable position memory that memorizes an allowable position range relevant to the pusher; an allowable reaction force memory that memorizes an allowable reaction force range relevant to the pusher; and a determiner that determines whether a length of the bound material is normal or abnormal; wherein the pusher moving device is configured to stop the pusher when the reaction force of the pusher arrives at an upper limit of the allowable reaction force range while the pusher is moved from the set standby position toward the set stop position; and wherein the determiner determines that: the length of the bound material is normal when the stopped position of the pusher is in the allowable position range, and the reaction force of the pusher is in the allowable reaction force range; or the length of the bound material is abnormal when the stopped position of the pusher is out of the allowable position range, or the reaction force of the pusher is out of the allowable reaction force range. the length of the bound material is normal when the stopped position of the pusher is in the allowable position range, and the reaction force of the pusher is in the allowable reaction force range; or the length of the bound material is abnormal when the stopped position of the pusher is out of the allowable position range, or the reaction force of the pusher is out of the allowable reaction force range.