A method for converting sample rates includes obtaining coefficients from a sample rate conversion coefficient table. The objective comprises a first diffractive structure and in which the diffraction order for the maximum diffraction efficiency is the same for all of light fluxes, and a second diffractive structure that diffracts the second light flux without diffracting the first and third light fluxes. The second position is contained in a range extending in the predetermined direction and covering the position of the maintenance target with the range being narrower than the first range. Then, with the same floating platform, the drilling riser is disconnected and moved to a second well.