A blockage determination portion 174 of a blockage detection system 30 determines that no blockage has occurred in a purge line 90, if a flow rate V of an inner fluid Fi exceeds a flow rate threshold THv while an internal pressure Pi of the purge line 90 is reduced from a first pressure value P1 to a second pressure value P2, for example. The blockage determination portion 174 determines that a blockage has occurred in the purge line 90, if the flow rate V of the inner fluid Fi falls below the flow rate threshold THv after the internal pressure Pi is reduced from the first pressure value P1 to the second pressure value P2.