A high-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.