A method for manufacturing an optical device is provided. The device includes an optical waveguide path having a Bragg grating and a movable portion disposed near the Bragg grating. A displacement of the movable portion provides a change of spacing of the Bragg grating so that a light passing through the optical waveguide path is changed. The optical device detects the physical quantity based on a change of the light. The method includes steps of: forming the optical waveguide path with the Bragg grating on a first part of a silicon substrate; and forming the movable portion on a second part of the silicon substrate.