An ablation needle device for performing a high-frequency ablation treatment of a tumor, comprising: a hollow needle that is made of a metal and that is composed of a proximal end portion having an outer surface that is insulation-coated with a resin and a distal end portion having an outer surface that is exposed and thus constituting an electrode; a hub that is attached to a proximal end side of the hollow needle and that includes a liquid injection port for injecting liquid for cooling the electrode and supplying the liquid to an inside of the hollow needle, an electric connector that is electrically connected to the hollow needle in order to supply a high-frequency electric current to the electrode; a thermocouple that extends in the inside of the hollow needle in order to measure a temperature of a tissue around the electrode; wherein the hollow needle is given flexibility by forming a helical slit in at least a distal end region of the proximal end portion of the hollow needle and the distal end portion, wherein a liquid-tightness of the inside of the hollow needle is ensured by applying waterproofing to an inner surface of the hollow needle in at least a region in which the slit is formed in the distal end portion, wherein the ablation needle device is characterized by the hub further including a liquid discharge port for discharging liquid that has cooled the electrode and returned from the inside of the hollow needle; wherein the liquid-tightness of the inside of the hollow needle is further ensured by closing a distal end of the hollow needle, and wherein the ablation needle device further comprises a plurality of cooling liquid introducing pipes (51,52)each of which extends in an inside of the hub and the inside of the hollow needle, each of which has a distal end positioned in an inside of the distal end portion of the hollow needle, and each of which ejects liquid injected from the injection port from a distal end opening thereof, wherein distal end opening positions of the plurality of cooling liquid introducing pipes differ from each other in a distal-proximal direction of the hollow needle. a hollow needle that is made of a metal and that is composed of a proximal end portion having an outer surface that is insulation-coated with a resin and a distal end portion having an outer surface that is exposed and thus constituting an electrode; a hub that is attached to a proximal end side of the hollow needle and that includes a liquid injection port for injecting liquid for cooling the electrode and supplying the liquid to an inside of the hollow needle, an electric connector that is electrically connected to the hollow needle in order to supply a high-frequency electric current to the electrode; a thermocouple that extends in the inside of the hollow needle in order to measure a temperature of a tissue around the electrode; wherein the hollow needle is given flexibility by forming a helical slit in at least a distal end region of the proximal end portion of the hollow needle and the distal end portion, wherein a liquid-tightness of the inside of the hollow needle is ensured by applying waterproofing to an inner surface of the hollow needle in at least a region in which the slit is formed in the distal end portion, wherein the ablation needle device is characterized by the hub further including a liquid discharge port for discharging liquid that has cooled the electrode and returned from the inside of the hollow needle; wherein the liquid-tightness of the inside of the hollow needle is further ensured by closing a distal end of the hollow needle, and wherein the ablation needle device further comprises a plurality of cooling liquid introducing pipes (51,52)each of which extends in an inside of the hub and the inside of the hollow needle, each of which has a distal end positioned in an inside of the distal end portion of the hollow needle, and each of which ejects liquid injected from the injection port from a distal end opening thereof, wherein distal end opening positions of the plurality of cooling liquid introducing pipes differ from each other in a distal-proximal direction of the hollow needle.