An etch-stop layer is selectively provided between layers of a multiple-layered circuit in a selective manner so as to allow for outgassing of impurities during subsequent fabrication processes. A first substrate having a porous layer inside, a single-crystal Si layer on the porous layer, and an SiO2 layer on the single-crystal Si layer is bonded to a second substrate. A reference modulation symbol set corresponding to a mini unit by modulating the sequence is generated.