This light source 1 is provided with a luminescent cylinder 3A housing a luminescent part 2 to generate light; a light guide cylinder 3B connected to the luminescent cylinder 3A on a one end side, and configured to guide the light generated by the luminescent part 2, to an exit window 4 provided on the other end side; and a cylindrical reflective cylinder 9 inserted and fixed between the exit window 4 of the light guide cylinder 3B and a portion connecting the luminescent cylinder 3A and the exit window 4, and having an inner wall surface as a reflective surface 9a to reflect the light.