Fixing device provided with heater and image forming apparatus

A fixing device for fixing an unfixed toner image on a recording material to the recording material, the fixing device comprising:

• a first rotatable member;
• a heater provided with a first heat generating member on a substrate and a second heat generating member of which a length in a longitudinal direction is shorter than a length of the first heat generating member;
• and configured to heat the first rotatable member;
• a first temperature detecting unit configured to detect a temperature of the heater;
• a second rotatable member configured to form a nip portion with the first rotatable member;
• a switching unit configured to switch a power supply path from an AC power source to the first heat generating member or the second heat generating member;
• a control unit configured to control the switching unit to supply power to the first heat generating member or the second heat generating member,
• wherein the control unit executes a first control in which the switching unit supplies the power to the first heat generating member, and switches a second control in which the switching unit supplies the power alternately to the first heat generating member or the second heat generating member when the temperature detected by the first temperature detecting unit reaches a threshold temperature.