A microwave surgical instrument, comprising: a microwave transmitting section; and a coaxial body, wherein the microwave transmitting section includes a coaxial cable including an internal conductor, wherein the coaxial body includes a central conductor, an insulator covering a part or a whole of the central conductor, and an external conductor connected to the microwave transmitting section and the internal conductor and covering a part or a whole of the insulator, wherein the coaxial body further includes one or a plurality of biological tissue contact sections each including the central conductor that is partially or fully covered with the insulator and the external conductor, wherein the external conductor is exposed in a major axis direction, a support adapted to support the coaxial body; characterised in that: the coaxial body is tapered, and a sectional shape of the central conductor of the tip of the coaxial body, and a sectional shape of the external conductor of the tip of the coaxial body are circular shapes, and a ratio between a diameter of the central conductor of the tip of the coaxial body and a diameter of the central conductor at the portion contacted to the microwave transmitting section is the same as a ratio between an inner diameter of the external conductor of the tip of the coaxial body and an inner diameter of the external conductor of the portion contacted to the microwave transmitting section, or a sectional shape of the central conductor of the tip of the coaxial body and a sectional shape of the external conductor of the tip of the coaxial body are not circular shapes, and a ratio between a sectional area of the central conductor of the tip of the coaxial body and a sectional area of the central conductor of the portion contacted to the microwave transmitting section is the same as a ratio between a sectional area of the external conductor of the tip of the coaxial body and a sectional area of the external conductor of the portion contacted to the microwave transmitting section. a microwave transmitting section; and a coaxial body, wherein the microwave transmitting section includes a coaxial cable including an internal conductor, wherein the coaxial body includes a central conductor, an insulator covering a part or a whole of the central conductor, and an external conductor connected to the microwave transmitting section and the internal conductor and covering a part or a whole of the insulator, wherein the coaxial body further includes wherein the microwave transmitting section includes a coaxial cable including an internal conductor, wherein the coaxial body includes a central conductor, an insulator covering a part or a whole of the central conductor, and an external conductor connected to the microwave transmitting section and the internal conductor and covering a part or a whole of the insulator, wherein the coaxial body further includes one or a plurality of biological tissue contact sections each including the central conductor that is partially or fully covered with the insulator and the external conductor, wherein the external conductor is exposed in a major axis direction, a support adapted to support the coaxial body; the coaxial body is tapered, and a sectional shape of the central conductor of the tip of the coaxial body, and a sectional shape of the external conductor of the tip of the coaxial body are circular shapes, and a ratio between a diameter of the central conductor of the tip of the coaxial body and a diameter of the central conductor at the portion contacted to the microwave transmitting section is the same as a ratio between an inner diameter of the external conductor of the tip of the coaxial body and an inner diameter of the external conductor of the portion contacted to the microwave transmitting section,