PURPOSE: To eliminate the effects of the infrared rays when the frequency of a surface acoustic wave element is controlled, by using an infrared ray cutting filter and at the same time cooling a water solution of copper sulfate which passes through said filter.

CONSTITUTION: An infrared ray cutting filter through which a water solution 5 of copper sulfate is circulated is put between two parallel flat plates 4 which transmit the ultraviolet rays. The ultraviolet rays sent from an ultraviolet ray lamp 1 are condensed by an oval mirror 2 and irradiated to a surface acoustic wave element 3 via an infrared ray cutting filter. Here the solution 5 passed through said filter is cooled by an external cooling device 6 through pipes 7 and 8 and circulated. Thus it is possible to eliminates the effects of the infrared rays when the frequency of the element 3 is controlled.