PURPOSE: To easily estimate the lifetime of a cold trap with accuracy by fixing a flow meter to an outlet portion of a small tube piercing through the capturing section and the tube forming a coolant pass for housing the section.

CONSTITUTION: A coolant such as metallic Na changing its flow direction by about 180° at a lower portion of outer wall 4 of cooling section 7 separately rises through capturing section 6 and small tube 9. Section 6 is composed of wire nets, etc., and precipitated impurities such as sodium oxide attach to the nets. With the passage of working time of the trap the amount of impurities attaching to the nets increases, and the area of the metallic Na pass in section 6 is reduced, so that flow rate of metallic Na rising through section 6 is reduced. Na corresponding to the reduced flow rate rises through tube 9, and it is measured with small tube flow meter 8. Accordingly, by measuring an increase in the amount of metallic Na rising through tube 9 with meter 8 the lifetime of the cold trap can be estimated easily.