A light source device comprising: an excitation light source configured to emit irradiation light of a color component to be used as excitation light and projection light; a fluorescent member disposed in an excitation light path and configured to generate fluorescent light when being irradiated with the irradiation light, the fluorescent light having a color component different from the color component of the irradiation light; a light path merger unit configured to merge a fluorescence light path through which the fluorescent light generated by irradiation of the irradiation light is delivered and a projection light path through which the irradiation light is delivered as the projection light; and a light path switch member configured to switch the irradiation light between the excitation light path and the projection light path, wherein the light path switch member includes a first reflective-transmissive area of a predetermined reflectance index which is configured to reflect a majority of the irradiation light and to transmit the rest of the irradiation light, so as to simultaneously guide one of the reflected and transmitted light to the excitation light path and the other of the reflected and transmitted light to the projection light path, so that the fluorescent light and the projection light are merged by the light path merging unit; characterized in that: the light path switch member further includes a second reflective-transmissive area configured to transmit a majority of the irradiation light and to reflect the rest of the irradiation light, so as to simultaneously guide the one of the reflected and transmitted light to the excitation light path and the other one of the reflected and transmitted light to the projection light path. an excitation light source configured to emit irradiation light of a color component to be used as excitation light and projection light; a fluorescent member disposed in an excitation light path and configured to generate fluorescent light when being irradiated with the irradiation light, the fluorescent light having a color component different from the color component of the irradiation light; a light path merger unit configured to merge a fluorescence light path through which the fluorescent light generated by irradiation of the irradiation light is delivered and a projection light path through which the irradiation light is delivered as the projection light; and a light path switch member configured to switch the irradiation light between the excitation light path and the projection light path, wherein the light path switch member includes a first reflective-transmissive area of a predetermined reflectance index which is configured to reflect a majority of the irradiation light and to transmit the rest of the irradiation light, so as to simultaneously guide one of the reflected and transmitted light to the excitation light path and the other of the reflected and transmitted light to the projection light path, so that the fluorescent light and the projection light are merged by the light path merging unit; characterized in that: the light path switch member further includes a second reflective-transmissive area configured to transmit a majority of the irradiation light and to reflect the rest of the irradiation light, so as to simultaneously guide the one of the reflected and transmitted light to the excitation light path and the other one of the reflected and transmitted light to the projection light path.