Ahigh-frequency treatment tool comprising: a flexible sheath to be inserted into a treatment tool insertion channel of an endoscope a treatment tool body provided inside the flexible sheath and comprising: a flexible cord and a high-frequency knife to which a high-frequency current is applied, formed to the front end of the flexible cord and a hard cylinder having an electrical insulating property, inserted and fixed inside the flexible sheath, wherein the high-frequency knife comprises blade portions radially provided in its axial direction, each of the blade portions including an engaging part, and wherein, when the high-frequency knife is made to stick out from the hard cylinder, engaging parts of the blade portions abut against a rear end portion of the hard cylinder so that: the high-frequency knife is engaged with the hard cylinder and communicating channels that communicate an inside and an outside of the flexible sheath are formed between the blade portions adjacent to each other.
