The advancing and retracting aid (100) has a fixing member (200) having a fixed side tubular part (205) through which a treatment instrument (51) can be inserted and a moving side tubular part (215) through which the treatment instrument And a moving member (210) in which the moving side tubular portion (215) is inserted into the fixed side tubular portion (205) or the fixed side tubular portion (205) is inserted into the moving side tubular portion (215). The advancing and retracting aid (100) is fixed to the fixed side tubular part (205), the moving side tubular part (215) and the fixed side tubular part (205) so as to ensure liquid tightness between the fixed side tubular part (205) and the moving side tubular part , And has a first elastic member (220) having a different diameter in the thickness direction and an engaging member (600) engaging the treatment instrument (51) with the moving side tubular portion (215).進退補助具(100)は、処置具(51)が挿通可能な固定側筒部(205)を有する固定部材(200)と、処置具(51)が挿通する移動側筒部(215)を有し、移動側筒部(215)が固定側筒部(205)に挿入されるまたは固定側筒部(205)が移動側筒部(215)に挿入される移動部材(210)とを有する。進退補助具(100)は、固定側筒部(205)と移動側筒部(215)との間の液密を確保するように固定側筒部(205)と移動側筒部(215)との間に配設され、厚み方向において径が異なる第1の弾性部材(220)と、処置具(51)を移動側筒部(215)に係合する係合部材(600)とを有する。
