PURPOSE: To prevent error operation even if the vacuum degree in an exhaust pipe is changed, by opening and closing a precut valve only when a vacuum degree detecting signal in the exhaust pipe and an ion detecting signal from an ion source are simultaneously detected.

CONSTITUTION: A solvent sent out from a column 3 is exhausted from an exhaust apparatus 5 but the not-exhausted solvent is introduced into an ion source 8 through a separator 7 and directly ionized to be emitted as ion beam. The part of this ion is detected by an ion detector 14 and this detected signal and the signal of a vacuum detecting element 11 are transmitted to a signal timing decision circuit 15 which in turn issues a pulse signal only when both signals are simultaneously transmitted and this pulse signal is transmitted to a timer 12 to open and close a precut valve 4. By this method, a signal is outputted from the circuit 15 only when a solvent is sent out from the column 3.