A laser light irradiation device is disclosed, which can include an optical guide having a columnar portion and transmitting laser light through the columnar portion; a reflecting portion having a reflecting surface that reflects the laser light transmitted through the optical guide; and a transmitting portion having a window part and disposed between the columnar portion of the optical guide and the reflecting portion in such a manner that one end of the transmitting portion is in contact with the columnar portion, the transmitting portion transmitting the laser light emitted from the columnar portion to the reflecting portion and transmitting the laser light reflected by the reflecting surface to cause the laser light to be radiated to an external through the window part. A diameter of the reflecting surface gradually becomes smaller as distance from a boundary between the transmitting portion and the reflecting portion increases.