An anti-leak valve unit (1) is removably attached to a base end portion (43) of an overtube (4) that includes an inner cavity (42) through which an endoscope is to be removably inserted, and that is to be inserted into a body cavity. The anti-leak valve unit (1) includes a frame portion (3) attached to an outer circumferential surface (44) of the base end portion (43) of the overtube (4), a fitting device (2) provided on the frame portion (3) so as to be removably engaged with the outer circumferential surface (44) of the base end portion (43), and a valve element (5) provided inside the frame portion (3). The valve element (5) includes an annular endoscope insertion port located at a generally central portion of the valve element (5) so as to be circumferentially fitted to the endoscope, and a flexible elastic portion provided around the endoscope insertion port.
