PURPOSE: To prevent external liquid from entering an elastic seal spool which engages a push rod by making the thickness of the elastic seal spool extremely thin.

CONSTITUTION: A guide member 2 and the elastic seal spool 3 which engage the push rod 1 are united and inserted into a heat sensing cylinder 5 where wax 4 is filled, and the upper part of the heat sensing cylinder 5 is pressed to constitute the thermo-actuator. Further, a snap ring 6 inserted into the groove of the push rod 1 prevents the push rod 11 from striking on the bottom of the seal spool 3. Then the thickness of the side wall of the center hole of the seal spool 3 which engages the push rod 1 is set to 0.3 - 0.05% of the diameter of the push rod 1. Consequently, when negative pressure is generated, the internal wall of the seal spool 3 contacts the push rod 1 suctionally, so external liquid is prevented completely from entering the spool.