In assembling a binocular loupe device, a loupe of the binocular loupe device is attached to an appropriate position at a correct angle according to a pupil position of a user in a near vision state. A rectangular, transparent measurement assisting plate fixed to a frame before attachment of a loupe is worn by a user, and the face of the user in a near vision state is photographed by a camera from a work object point P to obtain a first image. The first image is subjected to arithmetic processing by a computer to measure, on the first image, coordinate positions of respective four vertexes of the measurement assisting plate and coordinate positions of respective left and right pupils of the user in the near vision state, and determines the loupe attachment position and/or loupe attachment angle on a plane of the plate member from the measured coordinate positions.