It is an object to provide a surgical instrument capable of locally applying microwaves to a minute biological tissue. It has been found that microwaves can be transmitted to a tip of a tapered coaxial body (9) and that microwaves are radiated from the entire central conductor exposed in a major axis direction by decreasing a sectional area (preferably diameter) of a central conductor (1) and a sectional area (preferably inner diameter) of an external conductor gradually or in a step-by-step manner with a ratio between the sectional area (diameter) of the central conductor (1) and the sectional area (inner diameter) of the external conductor being set to be constant, thereby achieving the present invention.
