There is provided a fixing device for thermally fixing a developing agent image to a sheet. The fixing device includes a tubular flexible member, a heater, a nip member, a reflection plate, a backup member, a stay and a temperature sensor. The flexible member has an inner peripheral surface defining an internal space. The heater is disposed in the internal space and is configured to generate a radiant heat. The nip member is disposed in the internal space, the inner peripheral surface being in sliding contact with the nip member. The reflection plate is configured to reflect the radiant heat from the heater toward the nip member, the reflection plate having an outer profile. The backup member is configured to provide a nip region in cooperation with the nip member for nipping the flexible member between the backup member and the nip member. The stay covers the reflection plate and supports the nip member, the stay having a profile in conformance with the outer profile of the reflection plate, and the stay being formed with one of a through-hole and a notch. The temperature sensor is disposed in the internal space and extends through the one of the through-hole and the notch.