A switching mechanism 110 can perform switching to a pressurized state in which gas is supplied from a pipe 203 to an insertion tube 101, or a derivation state in which gas in a head space 23 that is pressurized is derived from the insertion tube 101 to the pipe 207 via a collection unit 104. The switching mechanism 110 includes a discharge valve 103 that puts the insertion tube 101 and the pipe 207 into a non-communication state in the pressurized state and puts the insertion tube 101 and the pipe 207 into a communication state in the derivation state. A resistance pipe 206 supplies gas to a channel between the collection unit 104 and the discharge valve 103 in the derivation state.