PURPOSE: To obtain a sufficiently thick quench hardened layer by forming a cooling jacket so that a cooling fluid is obliquely sprayed in all directions of the circumference including the outer periphery of a heating conductor section.

CONSTITUTION: A cooling jacket 2 is coaxially arranged at a preset interval on the extension line of the center axial line O of the heating conductor section 1a of a coil 1, it is a trapezoidal hollow box with its outer diameter set thoroughly smaller than the inner diameter of a cylinder W, multiple small holes (s) are bored on the slant face 21 of its upper periphery, and the cooling fluid fed into the box 2 via a cooling fluid feed pipe P is sprayed obliquely upward in all directions of the circle including the outer periphery of the heating conductor section 1a as shown by arrows. The coil 1 and the cooling jacket 2 are fixed by a support member 4 via leads R and R and the pipe P, and they are integrated as a quenching device 10. The quenching device 10 movable in the vertical directions (a) and (b) and the right and left directions (d) and (c) is located at the position O, it coincides with the axis Ow of the cylinder W, the cooling fluid is sprayed through an injection hole S for cooling.