An image forming apparatus includes 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, and a controller configured to determine a first amount of electric power to be supplied to the heating element based on the temperature of 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 start rotation.