Disclosed herein are a short arc type flash lamp having high lamp starting performance and capable of reducing the diameter of its seal tube part, and a light source device thereof. The flash lamp has an electrode shaft of one of the main electrodes, and an electrode shaft of the other of the main electrodes and leads for starting auxiliary electrodes which are respectively led out from the second seal tube part, and an external trigger is disposed in a state in which it extends in the circumferential direction on the outer peripheral surface of one end side region of the second seal tube part. The light source device is structured by a concave reflection mirror disposed on the second seal tube part side of the flash lamp in a state in which a focal point of the concave reflection mirror coincides with a luminous point of the flash lamp.