A low-heat loss, belt nip-type fixing device achieves good recording medium separation performance without increasing the size of the device, prevents image misalignment, and achieves a high-quality image without placing a major burden on the recording medium. This fixing device comprises a heat-fixing roller having an elastic layer and a heat source; and a pressurizing unit having an elastic member with a flat surface for pressing an endless member. The endless member conveys a recording medium, against the elastic layer. A pressurizing spring biases the elastic member toward the side of the recording medium. Biasing is carried out using the pressurizing spring such that the amount of deformation of the elastic member approaches the saturation region (maximum amount of deformation) inside a region of a nip portion, which is formed between the elastic layer of the heat-fixing roller and the endless member of pressurizing unit, and through which is passed a recording medium carrying an unfixed toner image.