The present invention relates to a light source apparatus or the like provided with a structure that can block reflected return light also when an emitting end side of an optical isolator is a space. The light source apparatus comprises a light source section, a guide section, and an optical module. The optical module includes a collimator, an optical isolator, and an oblique-beam blocking section. The collimator outputs collimated light with a predetermined beam diameter as a forward propagating beam. The optical isolator is a polarization-independent optical isolator that introduces the collimated light from a first end and outputs this collimated light from a second end. The oblique-beam blocking section includes first and second optical components each having a window of a diameter D, and blocks light incident at a predetermined angle relative to an optical axis of the forward propagating beam out of a backward propagating beam, by separating the first and second optical components from each other by a predetermined distance.