A thin board holding device 2A for holding a thin board in a plane state to measure the thickness of a thin board is disclosed, which has a number of spot supports 3 with adsorbing portion for holding a thin board 1 attached by suction to the top. The thin board 1 is held in a multiple point support fashion to the spot supports 3. By moving a measuring terminal of a measuring instrument to a measuring point on the thin board, which has one surface of the thin board 1 set on the thin board holding device 2A, the level of the other surface of the thin board 1 from a reference surface A supporting the thin board holding device at a reference position is measured. The dimension from the reference position A to a thin board adsorption surface 3a of the spot support 3 is used as a computational constant, and the thickness of the thin board is calculated from the measured level of the other surface of the thin board 1 and the computational constant.