PURPOSE: To prevent decrease of recovery efficiency of a condensed material due to bumping with a simple structure apparatus, by inserting an insertion part of bumping preventing valve into the lower end of a rotary joint and raising and sucking a valve plate to the end of the valve body by utilizing the pressure difference in the time of strong bumping.

CONSTITUTION: Usually, the pressures of the inside of a sample flask 31 and the inside of a cooling apparatus 32 are constant in an equilibrium state. Once bumping occurs in heating a sample soln., the inside pressure of the flask 31 becomes positive pressure and the inside pressure of the cooling apparatus becomes negative pressure, and due to the difference of the pressures a valve plate of a bumping preventing valve 1 is raised and sucked to the lower end of the valve body 2. As a result, the lower end of the valve body 2 is closed and a strong bumping is interrupted, therefore, loss of the sample soln. in the flask 31 is prevented. When the strong bumping ends, a leak-cock 37 is pulled out so as to lead air and return the valve plate 12 to the original position. In case the valve body 2, a support 3, the valve plate 12, etc., of the bumping preventing valve 1 are made of tetrafluoro resin, resistance to heat and chemicals are increased.