An image forming apparatus comprising: a fixing unit including: a heater including a heating element, a fixing belt having a surface to which a lubricant is applied and contacting the heater through the lubricant, and a pressing roller capable of pressing and rotating the fixing belt; a power supply configured to supply electric power to the heating element; a first thermometer configured to measure temperature of the heating element; a second thermometer configured to measure temperature of the fixing belt; and a controller configured to - determine a first amount of electric power to be supplied to the heating element, and control the power supply to supply the determined first amount of electric power to the heating element, before controlling the pressing roller to rotate, - determine the first amount of electric power based on the temperature of the heating element and the temperature of the fixing belt, and - determine, as the first amount of electric power, a first predetermined amount of electric power when the temperature of the fixing belt is lower than a first predetermined value and the temperature of the heating element is lower than a second predetermined value, characterized in that the controller is configured to determine, as the first amount of electric power, a second predetermined amount of electric power when the temperature of the fixing belt is lower than the first predetermined value and the temperature of the heating element is equal to or higher than the second predetermined value but lower than a third predetermined value, and that the second predetermined amount of electric power is less than the first predetermined amount of electric power. a fixing unit including: a heater including a heating element, a fixing belt having a surface to which a lubricant is applied and contacting the heater through the lubricant, and a pressing roller capable of pressing and rotating the fixing belt; a heater including a heating element, a fixing belt having a surface to which a lubricant is applied and contacting the heater through the lubricant, and a pressing roller capable of pressing and rotating the fixing belt; a power supply configured to supply electric power to the heating element; a first thermometer configured to measure temperature of the heating element; a second thermometer configured to measure temperature of the fixing belt; and a controller configured to - determine a first amount of electric power to be supplied to the heating element, and control the power supply to supply the determined first amount of electric power to the heating element, before controlling the pressing roller to rotate, - determine the first amount of electric power based on the temperature of the heating element and the temperature of the fixing belt, and - determine, as the first amount of electric power, a first predetermined amount of electric power when the temperature of the fixing belt is lower than a first predetermined value and the temperature of the heating element is lower than a second predetermined value, characterized in that - determine a first amount of electric power to be supplied to the heating element, and control the power supply to supply the determined first amount of electric power to the heating element, before controlling the pressing roller to rotate, - determine the first amount of electric power based on the temperature of the heating element and the temperature of the fixing belt, and - determine, as the first amount of electric power, a first predetermined amount of electric power when the temperature of the fixing belt is lower than a first predetermined value and the temperature of the heating element is lower than a second predetermined value, the controller is configured to determine, as the first amount of electric power, a second predetermined amount of electric power when the temperature of the fixing belt is lower than the first predetermined value and the temperature of the heating element is equal to or higher than the second predetermined value but lower than a third predetermined value, and that the second predetermined amount of electric power is less than the first predetermined amount of electric power.