A blood-vessel optical device is provided with: a sheath provided with a lumen penetrating therethrough, and a dovetail groove disposed so as to be parallel to the lumen； and an optical member accommodated in the lumen and including at least one of an illuminating portion guiding illumination light to an imaging subject and a light detecting portion guiding return light from the imaging subject, wherein the groove has a cross-sectional shape configured to accommodate an elongated member so as to be movable, and has an opening that has a width smaller than a diameter of the elongated member and from which a portion of the accommodated elongated member protrudes to the outside, and wherein the sheath is configured such that an external dimension in a direction orthogonal to an arrangement direction in which the lumen and the groove are arranged is smaller than an external dimension in the arrangement direction.
