An illumination light source device includes a reflection-transmission wheel provided on a light-path from an excitation light source and having a transmission region that transmits an excitation light and a reflection region reflecting the excitation light, a fluorescent wheel provided on at least one of the reflection light-path and the transmission light-path and having a fluorescent body emitting fluorescence when excited by the excitation light, and a control section turning-on the excitation light source while the boundary of the transmission region and the reflection region traverses the light-path, in order to provide an illumination light source device capable of preventing a deterioration of the fluorescent body, whereby improving a color reproducibility by use of the color mixture occurred at the boundary region between the transmission light-path and the reflection light-path effectively and improving the brightness of image.