An automated ac susceptibility measuring system based on an ac mutual inductance bridge is described. A personal computer and a digital‐analog converter are used to control the amplitude and phase of the compensatory signal which balances the bridge. An analog multiplier and a variable‐resistance photocoupler are used in the amplitude and phase control circuits, respectively. The magnetic moment of a sample for a 1‐&mgr;V output voltage is 3.8×10−6emu/&mgr;V. Sufficient sensitivity was confirmed for an application to oxide superconductors.