PURPOSE: To prevent the degree of ultimate vacuum from being reduced due to water absorption by oil in evacuation at the use of a rotary pump and enable vapor to be simply exhausted by heating and evaporating water in a cryopump by means of a heater and exhausting the vapor together with dry gas to the outside.

CONSTITUTION: After stopping the operation of a cryopump, respective heaters 13, 17 are heated, while with the opening of a valve 15 dry gas such as nitrogen, etc., is introduced into a case 2. And, upon checking, by an atmosphere checking instrument 11 that pressure in the case 2 reaches atmospheric pressure, a valve 9 on a vapor exhausting passage 10 is opened by a signal from the checking instrument 11 or manually. Then, the rise of temperature in the case 2 over 0°C causes ice to be melted to change to liquid. And, said liquid falls onto a moisture evaporating part 14, vapor evaporated from whose evaporating part 14 is exhausted with the dry gas to the outside through the exhaust passage 10. Hereby, it is made possible when there is provided much water at the evacuating operation of a conventional rotary pump, that oil contained in vacuum exhaust gas absorbs water to previously prevent troubles such as bad ultimate vacuum, etc., from being produced and to singly exhaust the vapor.