The invention relates to a light source device and a projection displaying device. In the light source device in which output light of a semiconductor laser device is focused and irradiated to a fluorescent member for exciting the fluorescent member and making the fluorescent member emit light, a distance between the fluorescent member and a focusing lens set can be hardly shortened, thereby causing low collecting efficiency of the diffused fluorescent light, and reducing brightness and power efficiency of the projection displaying device. Furthermore, the external dimension of a rotating member which is coated by the fluorescent member is larger than that of motor, and furthermore the rotating axis of the rotating member is parallel with the optical axis of the focusing fluorescent lens set, thereby causing large dimension of the device. According to the light source device and the projection displaying device, the optical axis of the focusing lens set is crossed with the rotating axis of the rotating member at an angle which is larger than 10 DEG and smaller than 80 DEG, and furthermore the fluorescent member is arranged in an oblique surface which is oblique relative to the rotating shaft of the rotating member. Because the distance between the fluorescent member and the focusing lens set can be reduced, more fluorescent light can be collected through the focusing lens set, thereby improving brightness and power efficiency of the projection displaying device and realizing smaller size of the device.