PURPOSE: To eliminate the need for any temp. sensor and control temperatures highly accurately by the constitution of controlling the electric power of a heating element by detecting the elongation and contracting owing to the temp. change of the free end of the temp. detecting bar provided in a heat roller by means of a detecting piece.

CONSTITUTION: When a heater 2 is energized by closing of a switch, the temp. of a heat roller 1 rises, and along with this, the temp. of a temp. detecting bar 10 rises as well. With this increase in temp., the size of the bar 10 elongates increasingly and when its free end pushes a detecting piece 12, the piece 12 rotates around a fulcrum shaft 11 and operates a switch 9 at the other end, thereby interrupting the supply of electric power to the heater 2. Then, the temp. of the heat roller 1 lowers and the temp. of the bar 10 lowers as well. Also, the piece 12 returns to the original position and the contacts of the switch 9 close, so that the heater 2 is again energized. By the repetition of these operations, the temp. of the heater 2 is controlled.