Alight source device including an arc tube having a light emission portion containing electrodes, a cooling channel having a downstream part branched into first and second channels to guide air toward the light emission portion via one of the channels, a rotational member disposed within the cooling channel and rotatable around a rotation shaft by gravity, and configured to switch cooling to the first or second channel depending on a position of the light source device, and a duct that forms apart of the cooling channel. The width of the rotational member decreases from a remote end toward the rotation shaft. The rotation shaft is disposed on one end of the rotational member corresponding to a side opposite a light emission side of the duct. The duct is shaped such that the cooling channel width decreases toward the side opposite the light emission side.