Abstract: A flat monolithic accelerometer detector comprises a body (12) having a base (16) and two measurement cells each having a seismic mass (24a-24b) connected to the base via a joint enabling the mass to turn about an axis perpendicular to a sensing axis of the detector and each having a vibrating beam force sensor connecting the mass to the base, the cells being placed in such a manner that when one of the beams is subjected to a traction force due to an acceleration along the sensing axis, the other beam is subjected to a compression force of the same magnitude, the cells being disposed in opposite directions and symmetrically about an axis of the base for fixing to a support (18) whose acceleration is to be measured. Each beam is constituted by at least two parallel blades (26, 28) that are at different distances from the joint, with the two blades in a given cell being connected to the seismic mass of that cell via a common hinge.