a beam having a base material of silicon monocrystal and at least one projection which is integrally formed so as to be supported at least at one end thereof and which has two surfaces having an orientation plane , includes a bottom surface in a plane which is common with a plane of the base material ; a groove penetrating from the bottom surface to a top of the projection ; and a protecting member having a resistance property against a crystal anisotropic etching liquid and covering an inner wall of the groove .