There are provided an optical member, an illuminating device, and a projection type video display apparatus, capable of satisfying both or at least one of the following two functions. That is, one is to prevent light re-incident upon a reflective polarizer from becoming linearly polarized light having an undesirable polarization direction. The other is to improve exploiting efficiency of returned light. A reflection member and a ¼λ plate are disposed on the light entrance surface side of a rod integrator, and a reflective polarizer is disposed on the light exit surface side thereof. The reflection member is formed with a light transmission-use aperture, and an LED chip of the LED is positioned in the light transmission-use aperture. A mirror is formed at the rear surface of the LED chip, thereby eliminating occurrence of light leakage from the light transmission-use aperture. Furthermore, provision of the above-described ¼λ plate prevents the light re-incident upon the reflective polarizer from becoming linearly polarized light having an undesirable polarization direction.