An optical device includes a light guide member, a holding member to hold the light guide member, an optical element to function according to light applied from the light guide member, a holder to hold the optical element, and an adhesive to join the holding member to the holder. The holding member and the holder are engaged with each other so that the optical element is placed on an optical axis of the light guide member. At least one of the holding member and the holder has a joint structure extending along the light guide member. The joint structure forms a gap to allow the adhesive to be easily injected between the holding member and the holder. The holding member and the holder are joined to each other with the adhesive injected into the gap.