A space through which a cooling fluid flows is provided in the inside of a connection portion between an endoscope and a light source device in order to cool heat generated in the connection portion. For example, a device for performing air supply or air suction (at least either one of air supply and air suction) in the endoscope communicates with the space, and cooling is effected by the flow of air formed by the device. In a preferred example of a cooling method, when neither air supply nor air suction is needed during manipulation of the endoscope, the device and the space are connected to each other to effect cooling, and when air supply or air suction is needed, the flow of air used for cooling is intercepted, and is exclusively directed to an air channel in the endoscope.