A surgical instrument for fastening together opposing portions of a body organ. The instrument comprises a first support for supporting the first opposing portion of the body organ, a second support opposing the first support, for supporting the second opposing portion of the body organ, and a displacement-preventing member connected to the first or second support for preventing the supports from displacing relative to each other. The member comprises a pin protruding from one of the supports and a hole formed in the other of the supports. The pin is made of thermoplastic resin. When heated, the pin is softened and deformed, whereby the first and second supports are secured to each other, fastening together the opposing portions of the body organ.