A light source for a photographic printer which is designed so that almost all light energy emitted from a diffusing device can be utilized for printing, thereby markedly improving the light source efficiency. The light source is used in a photographic printer which illuminates a film (F) and projects an image of the film (F) onto photographic paper (P) to print the projected image on the paper (P). A diffuser (BD), which is disposed in an illuminating optical path in the vicinity of the film (F), is caused to perform reciprocative vibration in parallel to its own plane by a vibrating device (B), e.g., a voice coil, during the exposure time, so that, even if an image of dust or a blemish on the diffuser (BD) is formed on the print, the image becomes blurred on the photographic paper (P) because it moves during the exposure process, thereby preventing the print from being affected by the image of such an undesired object. Thus, it is possible to place the film (F) and the diffuser (BD) closer to each other than in the case of the conventional printer light sources. Consequently, the quantity of light that can be utilized for printing can be increased to a considerable extent.