Provided is a surgical instrument that is substitutable for an operation of placing each stitch with a threaded needle or a stapler. It is confirmed that, in a tissue suturing device including a clamping section, and a protruding section including a central conductor and/or an external conductor for applying a microwave, the protruding section and the clamping section bring fragmentary portions of tissues to be sutured into contact or overlap with each other, and coagulate and/or fix contact or overlap portions with the microwave, to thereby stitch the tissues to be sutured.