A fixing device includes a contact portion that comes into contact with a recording material transported; a heat source that heats the contact portion and has a counter surface and an opposite surface, the counter surface facing the contact portion; a high-thermal-conductivity portion provided on the opposite surface of the heat source and extending in a width direction intersecting a transport direction of the recording material, the high-thermal-conductivity portion having a higher thermal conductivity than the contact portion; a low-thermal-conductivity portion provided between the opposite surface of the heat source and the high-thermal-conductivity portion and having a lower thermal conductivity than the high-thermal-conductivity portion; and a temperature detector that detects a temperature of the heat source and is provided on the opposite surface of the heat source and at a position shifted from the high-thermal-conductivity portion and the low-thermal-conductivity portion in the transport direction.