PROBLEM TO BE SOLVED: To provide a heating coil, a thermal treatment device and a thermal treatment method which are capable of high-speed thermal treatment for a portion of side faces of a long material with less power.SOLUTION: A heating coil 21 of a long material 10 for inductively heating the long material 10 is provided. In the long material 10, a first side face 11 and a second side face 12 are adjacent to each other at a corner 13, a recess 14 is disposed on the first side face 11 apart from the corner 13, and the first and second side faces 11, 12 and the recess 14 are continuous in a longitudinal direction. Along the longitudinal direction, there are disposed a wide conductor 24 that faces the first side face 11 and a narrow conductor 23 that faces the recess 14 and protrudes from the wide conductor 24 to the recess 14 side.