An energy treatment device (1, 1A) comprising: an insertion part which is capable of being inserted into a body; a pair of arms which are provided in a distal portion of the insertion part and which are capable of performing an opening-closing operation; a treatment member which is attachably and detachably provided in a first arm that is one of the pair of arms and which performs treatment on a treatment target site; a locked part (24, 24A) which is provided in a second arm that is different from the first arm among the pair of arms and which is capable of locking the treatment member; and a manipulation part which causes the pair of arms to perform the opening-closing operation, wherein the treatment member includes: a knife wire configured to have conductivity and to incise a biological tissue using electrical energy; and a locking member (12; 12A) which is positioned with respect to the first arm, being attachable to and detachable from the first arm and capable of being locked by the locked part (24; 24A), wherein the locked part (24; 24A) has a retainer which is capable of holding the locking member (12; 12A) with a force stronger than a force of the first arm to hold the locking member (12; 12A), and wherein the insertion part has a shaft to which each of the first arm and the second arm is connected so as to be swingable about a same or parallel axis lines respectively, wherein the first arm includes: a first proximal arm which is coupled to a distal end part of the shaft and which is configured to swing with respect to the shaft between a state in which the first proximal arm extends in parallel with a longitudinal axis of the shaft and a state in which the first proximal arm inclines to the longitudinal axis and extends radially outward from the shaft; and a first distal arm (7; 7A) which is coupled to the first proximal arm and which is configured to swing with respect to the first proximal arm between a state in which the first distal arm (7; 7A) extends in a direction in which the first proximal arm extends and a state in which the first distal arm (7; 7A) inclines to the direction in which the first proximal arm extends, wherein, after the first arm and the second arm are moved to a position in which the first arm and the second arm are in proximity to each other by the opening-closing operation, the locking member (12; 12A) held by the first arm is held by the retainer, and the locking member (12; 12A) is configured to detach from the first arm while the locking member (12; 12A) is held by the retainer of the second arm through an operation in which the first arm and the second arm are separated from each other. an insertion part which is capable of being inserted into a body; a pair of arms which are provided in a distal portion of the insertion part and which are capable of performing an opening-closing operation; a treatment member which is attachably and detachably provided in a first arm that is one of the pair of arms and which performs treatment on a treatment target site; a locked part (24, 24A) which is provided in a second arm that is different from the first arm among the pair of arms and which is capable of locking the treatment member; and a manipulation part which causes the pair of arms to perform the opening-closing operation, wherein the treatment member includes: a knife wire configured to have conductivity and to incise a biological tissue using electrical energy; and a locking member (12; 12A) which is positioned with respect to the first arm, being attachable to and detachable from the first arm and capable of being locked by the locked part (24; 24A), a knife wire configured to have conductivity and to incise a biological tissue using electrical energy; and a locking member (12; 12A) which is positioned with respect to the first arm, being attachable to and detachable from the first arm and capable of being locked by the locked part (24; 24A), wherein the locked part (24; 24A) has a retainer which is capable of holding the locking member (12; 12A) with a force stronger than a force of the first arm to hold the locking member (12; 12A), and wherein the insertion part has a shaft to which each of the first arm and the second arm is connected so as to be swingable about a same or parallel axis lines respectively, wherein the first arm includes: a first proximal arm which is coupled to a distal end part of the shaft and which is configured to swing with respect to the shaft between a state in which the first proximal arm extends in parallel with a longitudinal axis of the shaft and a state in which the first proximal arm inclines to the longitudinal axis and extends radially outward from the shaft; and a first distal arm (7; 7A) which is coupled to the first proximal arm and which is configured to swing with respect to the first proximal arm between a state in which the first distal arm (7; 7A) extends in a direction in which the first proximal arm extends and a state in which the first distal arm (7; 7A) inclines to the direction in which the first proximal arm extends, wherein, after the first arm and the second arm are moved to a position in which the first arm and the second arm are in proximity to each other by the opening-closing operation, the locking member (12; 12A) held by the first arm is held by the retainer, and the locking member (12; 12A) is configured to detach from the first arm while the locking member (12; 12A) is held by the retainer of the second arm through an operation in which the first arm and the second arm are separated from each other.