A mechanical magnitude sensor integrated on silicon, and a method of manufacture. The sensor comprises a bendably deformable conductive blade (4) whose free end (43) constitutes the first plate of a variable capacitor whose fixed second plate (24) is constituted by a conductive zone formed on the silicon substrate. A JFET type structure is formed in the vicinity of the anchor point (41) of the blade (4) with a gate zone (21) situated beneath the anchor portion (41) and with drain and source zones (22, 23) being provided on either side of the gate zone (21) in order to amplify a signal representative of variations in the position of the flexible blade (4). The sensor may be used as an accelerometer or as a pressure sensor.