An image forming apparatus has a printing section, a cooling mechanism, a suction adjustment mechanism, and a controller. The printing section includes a fixing unit for heating a sheet having a toner image put on it, and discharges the sheet after fixing. The cooling mechanism includes a suction port, a blower motor for rotating a fan, and a blowing duct for blowing air onto the sheet. The suction adjustment mechanism moves a suction adjustor between an open position where the suction port is open and a shut position where the suction port is closed. The controller moves the suction adjustor the closer to the shut position the smaller the set thickness of the sheet.