A cleaning auxiliary tool to be inserted into a bottomed cylinder having a bottomed cylindrical shape, the bottomed cylinder having a first opening through which a fluid from a first conduit is introduced, the first conduit being inserted through an endoscope, a second opening through which the fluid is led out into the first conduit, a third opening through which the fluid from a second conduit is introduced, the second conduit running in parallel with the first conduit in the endoscope, and a fourth opening through which the fluid is led out into the second conduit, the cleaning auxiliary tool comprising: a lid portion that closes an opening of the bottomed cylinder; a shaft portion having a first end portion connected to the lid portion and a second end portion extending into the bottomed cylinder; a partition portion that is provided on a side of the second end portion of the shaft portion and positioned between the first opening and the third opening and between the second opening and the fourth opening, and partitions a side of the first conduit and a side of the second conduit with a gap forming a space of a predetermined distance from an inner wall of the bottomed cylinder; and a close contact portion that is arranged on an outer circumferential side of the partition portion, is spaced apart from the inner wall of the bottomed cylinder by the predetermined distance when an internal pressure on the side of the first conduit corresponds to an internal pressure on the side of the second conduit, is deformed by receiving the internal pressure on the side of the first conduit and comes in close contact with the inner wall of the bottomed cylinder when the internal pressure on the side of the first conduit becomes higher than the internal pressure on the side of the second conduit, and is deformed by receiving the internal pressure on the side of the second conduit and comes in close contact with the inner wall of the bottomed cylinder when the internal pressure on the side of the second conduit becomes higher than the internal pressure on the side of the first conduit. a lid portion that closes an opening of the bottomed cylinder; a shaft portion having a first end portion connected to the lid portion and a second end portion extending into the bottomed cylinder; a partition portion that is provided on a side of the second end portion of the shaft portion and positioned between the first opening and the third opening and between the second opening and the fourth opening, and partitions a side of the first conduit and a side of the second conduit with a gap forming a space of a predetermined distance from an inner wall of the bottomed cylinder; and a close contact portion that is arranged on an outer circumferential side of the partition portion, is spaced apart from the inner wall of the bottomed cylinder by the predetermined distance when an internal pressure on the side of the first conduit corresponds to an internal pressure on the side of the second conduit, is deformed by receiving the internal pressure on the side of the first conduit and comes in close contact with the inner wall of the bottomed cylinder when the internal pressure on the side of the first conduit becomes higher than the internal pressure on the side of the second conduit, and is deformed by receiving the internal pressure on the side of the second conduit and comes in close contact with the inner wall of the bottomed cylinder when the internal pressure on the side of the second conduit becomes higher than the internal pressure on the side of the first conduit.