To provide a light quantity adjusting device capable of utilizing an infrared ray shielding filter highly reliably as required without becoming too thick.

The light quantity adjusting device includes: a light quantity adjusting mechanism which changes an opening area to adjust the quantity of light passing the opening; and a filter switching mechanism which moves an infrared ray shielding filter 43 to the opening position of the light quantity adjusting mechanism and separates the infrared ray shielding filter from the opening position, in which a recessed portion (housing space) 110b for housing the infrared ray shielding filter of the filter switching mechanism is provided on a separation member 110, which faces the infrared ray shielding filter, in such a manner that the infrared ray shielding filter is not interfered with the separation member.