An auxiliary instrument according to the present invention is an auxiliary instrument for dental use used as a reference of alignment for creating three-dimensional intraoral image data by connecting a plurality of pieces of three-dimensional image data obtained by three-dimensionally measuring an inside of an oral cavity, the auxiliary instrument including a sheet portion having a first surface and a second surface opposed to the first surface, and a plurality of identification portions formed in a thickness direction (Z direction) of the sheet portion from the first surface of the sheet portion, each of the plurality of identification portions having a three-dimensional shape configured to be identified.
