An infusion device comprises a pusher for a plunger of a syringe containing a liquid to be infused. A load cell measures the push force. An encoder associated to a motor commanding the pusher measures the displacement of the pusher. A controller signals an alarm when the ratio between the variation of the push force and the displacement exceeds a predetermined threshold. The device, which serves for infusing an anticoagulant into an extracorporeal blood circuit in a dialysis apparatus, is able to signal an onset of an anomalous situation of lack of infusion in good time.