A cartridge which accommodates a treatment device (60; 96; 98) comprising a coupling member (62; 96b; 98c) having at a proximal end an engagement portion which is engaged with a distal end of an introduction device which is used together with an endoscope and is introduced into a body cavity, and a treatment device main body (61, 63; 96a; 98a) provided at a distal end of the coupling member, the cartridge comprising: a treatment device accommodating portion which accommodates the treatment device, wherein the treatment device accommodating portion comprises: a coupling member accommodating portion which is provided at a proximal end of the treatment device accommodating portion, formed along an outer peripheral surface of the engagement portion of the treatment device, and holds the engagement portion in a state where a distal end of the introduction device is prevented from being engaged with the engagement portion of the coupling member (62; 96b; 98c); and a treatment device main body accommodating portion which has on a distal end side of the coupling member accommodating portion an engagement allowing portion which allows engagement of the engagement portion with respect to the distal end of the introduction device when a power is applied to move the engagement portion toward a distal end from a proximal end of the treatment device accommodating portion in case of engaging the engagement portion with the distal end of the introduction device. a treatment device accommodating portion which accommodates the treatment device, wherein the treatment device accommodating portion comprises: a coupling member accommodating portion which is provided at a proximal end of the treatment device accommodating portion, formed along an outer peripheral surface of the engagement portion of the treatment device, and holds the engagement portion in a state where a distal end of the introduction device is prevented from being engaged with the engagement portion of the coupling member (62; 96b; 98c); and a treatment device main body accommodating portion which has on a distal end side of the coupling member accommodating portion an engagement allowing portion which allows engagement of the engagement portion with respect to the distal end of the introduction device when a power is applied to move the engagement portion toward a distal end from a proximal end of the treatment device accommodating portion in case of engaging the engagement portion with the distal end of the introduction device. a coupling member accommodating portion which is provided at a proximal end of the treatment device accommodating portion, formed along an outer peripheral surface of the engagement portion of the treatment device, and holds the engagement portion in a state where a distal end of the introduction device is prevented from being engaged with the engagement portion of the coupling member (62; 96b; 98c); and a treatment device main body accommodating portion which has on a distal end side of the coupling member accommodating portion an engagement allowing portion which allows engagement of the engagement portion with respect to the distal end of the introduction device when a power is applied to move the engagement portion toward a distal end from a proximal end of the treatment device accommodating portion in case of engaging the engagement portion with the distal end of the introduction device.