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.