An illumination light source system includes an excitation light source; a reflection/transmission member having a transmission area which transmits excitation light from the excitation light source and a reflection area which reflects the excitation light, the transmission area and the reflection area sequentially crossing an emission light path of the excitation light source; a fluorescent member provided in at least one of a reflection light path of the excitation light reflected by the reflection area and a transmission light path of the excitation light transmitted through the transmission area, the fluorescent member being provided with a phosphor which generates a color different from that of the excitation light by excitation with the excitation light; and a controller which reduces a light volume of the excitation light source while a boundary area between the transmission area and the reflection area of the reflection/transmission member is located on the emission light path.