PURPOSE: To improve coagulating ability by changing at least one of generation interval and generation time of a high frequency output constituting a coagulation wave shape and thereby controlling the coagulation wave shape.

CONSTITUTION: A control circuit 2 is connected with a wave shape generating circuit 4 wherein informations on an output generation time T1, an output generation interval T2 and an output wave shape level are input. The wave shape generating circuit 4 is controlled by means of the control circuit 2 based on a condition set by an output setting means 3 to generate a specified wave shape, which is amplified by means of an amplifier 5 and is fed to the primary side of an output transformer 6. An output generated at the secondary side of the output transformer 6 is output from output terminals 8 and 8 through output capacitors 7 and 7. For example, a patient electrode and a treating electrode are connected with the output terminals 8 and 8. E.g. when a coagulation wave shape is generated, the control circuit 2 changes an output generating interval T2 to change an output electricity. E.g. when the output is small, even if the output generating interval T2 is large, a required output is small and it is thereby possible to output a set output, to make a crest factor extremely large and to make coagulation ability high.