A light source device which can prevent a large-sized structure of the device, a light source device which can improve assembling efficiency and maintainability of the device, and an image forming method and apparatus in which occurrence of moisture condensation can be prevented, are provided. An LED substrate in which a large number of light emitting diodes (LED) are arranged on the surface thereof in a two-dimensional manner, a base, a Peltier element, and a radiating fin are formed integrally by urging force of a compression spring. The radiating fin is disposed in contact with a light source housing body. Further, in carrying out temperature adjustment control using the Peltier element, the temperature adjustment control is carried out only when the internal temperature of the device in which a light source device is provided, is a predetermined temperature or higher.