To make thermal stress imposed on a heater small by preventing the position of a gap g from continuously coinciding with the position E where the edge part of a recording material passes.

This heat fixing device is provided with the heater 3, a thermistor 11 coming into contact with the heater 3 and a heater stay 1 which supports the heater 3 and has a thermistor disposing opening 1b where the thermistor 11 is disposed. This device is constituted so that a part of each of the thermister 11 and the stay 1 is brought into contact with the heater 3 at the position E where the edge part of the recording material passes on the heater 3.