An anti-leak valve unit is removably attached to a base end portion of an overtube through which an endoscope is removably insertable. The anti-leak valve unit includes a frame portion attachable to the overtube, a fitting device on the frame portion that is removably engageable with the base end portion, and a valve element provided inside the frame portion. The frame portion and the fitting device can be embodied as first through third interconnected covers and first and second members attached to the first cover at edges thereof. The valve element can be embodied as a flexible member with an annular endoscope insertion port in a flexible elastic portion thereof.