The present invention relates to a coil assembly consisting of the insert in the mounting portion to the large diameter of the product downward state into the large diameter of the product to heat the side of the product. Coil assembly for a high-frequency heat treatment according to the invention, inserted inside the large-diameter (S ') of the product (S) product (S) applies heat to the inner coil member 520 and; And the coil holder 530 which is provided on the lower side of the coil member (520) supporting the coil member (520); The coil is coupled to the top of the holder 530, the coil member core member 540 to control the heat emanating from the (520) and; Of the coil holder 530 and the lower binding, it can for guiding the horizontal movement of the coil member 520 is critically 502 and; Is provided on the upper side of the can critically 502, it consists of a large diameter (S ') lower nozzle for injecting cooling water on the inner surface 504 of the product (S) from the lower side. According to the present invention, there is an advantage that the anti-thermal deformation of the small-diameter and large-diameter parts of the product heat treatment efficiency is improved.