An intrauterine tissue collecting instrument of which a distal end portion is configured to be inserted into a uterus, the intrauterine tissue collecting instrument comprising: a flexible outer tube of which a distal end portion is configured to be inserted into the uterus; a flexible inner tube that is configured to be inserted into the flexible outer tube and is configured to be movable within the flexible outer tube in an axial direction; and a shaft member that is configured to be inserted into the flexible inner tube and is configured to be movable within the flexible inner tube in an axial direction, wherein the flexible outer tube includes an outer tube distal end portion that can be open, the flexible inner tube includes an inner tube distal end portion that includes a closed distal end and a plurality of side openings (34a-b, 35a-b) that are provided on a proximal end side with respect to the closed distal end, the shaft member includes a diameter-expanded distal end portion that is configured to slide within the flexible inner tube in a liquid-tight manner and a core, the shaft member has a higher resistance to movement toward the distal end side in the flexible inner tube than toward the proximal end side in the flexible inner tube, the flexible outer tube is configured to allow the inner tube distal end portion to protrude from the outer tube distal end portion, and the intrauterine tissue collecting instrument is configured to collect intrauterine tissue through suction via the side openings (34a-b, 35a-b) of the flexible inner tube when the shaft menber is moved toward the proximal end in a state where the inner tube distal end portion protrudes from the outer tube distal end portion within the uterus, and the inner tube distal end portion protruding from the outer tube distal end portion is configured to be housed again in the flexible outer tube within the uterus by moving the flexible inner tube toward the proximal end, characterized in that the distal end portion of the outer tube includes a substantially semispherical distal end and at least three slits that extend from substantially the center of the distal end toward a proximal end and in the direction of a side surface and are arranged at substantially equal angles with respect to a center axis of the flexible outer tube, wherein the inner tube is configured to spread the slits formed portion and thus is configured to open rprm the outer tube when the flexible inner tube is pushed and the diameter-expanded distal portion being a flare shaped diameter expanded portion of which the diameter increases toward a distal end, and the shaft member includes a shaft tube and the core that is housed and fixed in the shaft tube, the inside of the flare-shaped diameter-expanded portion is filled with resin. a flexible outer tube of which a distal end portion is configured to be inserted into the uterus; a flexible inner tube that is configured to be inserted into the flexible outer tube and is configured to be movable within the flexible outer tube in an axial direction; and a shaft member that is configured to be inserted into the flexible inner tube and is configured to be movable within the flexible inner tube in an axial direction, wherein the flexible outer tube includes an outer tube distal end portion that can be open, the flexible inner tube includes an inner tube distal end portion that includes a closed distal end and a plurality of side openings (34a-b, 35a-b) that are provided on a proximal end side with respect to the closed distal end, the shaft member includes a diameter-expanded distal end portion that is configured to slide within the flexible inner tube in a liquid-tight manner and a core, the shaft member has a higher resistance to movement toward the distal end side in the flexible inner tube than toward the proximal end side in the flexible inner tube, the flexible outer tube is configured to allow the inner tube distal end portion to protrude from the outer tube distal end portion, and the intrauterine tissue collecting instrument is configured to collect intrauterine tissue through suction via the side openings (34a-b, 35a-b) of the flexible inner tube when the shaft menber is moved toward the proximal end in a state where the inner tube distal end portion protrudes from the outer tube distal end portion within the uterus, and the inner tube distal end portion protruding from the outer tube distal end portion is configured to be housed again in the flexible outer tube within the uterus by moving the flexible inner tube toward the proximal end, characterized in that the distal end portion of the outer tube includes a substantially semispherical distal end and at least three slits that extend from substantially the center of the distal end toward a proximal end and in the direction of a side surface and are arranged at substantially equal angles with respect to a center axis of the flexible outer tube, wherein the inner tube is configured to spread the slits formed portion and thus is configured to open rprm the outer tube when the flexible inner tube is pushed and the diameter-expanded distal portion being a flare shaped diameter expanded portion of which the diameter increases toward a distal end, and the shaft member includes a shaft tube and the core that is housed and fixed in the shaft tube, the inside of the flare-shaped diameter-expanded portion is filled with resin.