An insertion instrument includes a shaft projecting, by being inserted through a hole of a fixing instrument, from a distal end of the fixing instrument toward a distal side, and a gas supply channel formed in the shaft section and having a jet port, which jets a gas, in a distal portion of the shaft section. The gas is supplied from a proximal side to the distal side toward the jet port in the gas supply channel, and an extension dimension along the longitudinal axis from the jet port toward the proximal side is greater than an extension dimension of the hole of the fixing instrument.
