To provide a device maintaining an in-furnace pressure of a heat treatment furnace to be constant.

A gas introduction chamber 8 having a port 9 introducing the gas exhausted from the heat treatment furnace is provided at the outer periphery of a bubbling tank 1 holding an oil liquid, and the chamber and the tank are made to communicate with each other by a gas outlet hole 1e provided in a boundary wall 1d between them and to hold the oil liquid in common. Inside the bubbling tank 1, a multihole plate 11 immersed in the oil liquid and disposed horizontally above the gas outlet hole 1e is provided.