A sensorless magnetic bearing type blood pump apparatus includes a blood pump and a control mechanism. The blood pump has electromagnets for rotating a rotating body in a non-contact condition, and a hydrodynamic bearing section for rotating the rotating body in a non-contact condition when operation of the electromagnets is stopped. The apparatus is devoid of any sensors for determining the position of the rotating body. The control mechanism includes a pulse width modulation type electromagnet driving unit, a carrier component measuring unit for measuring carrier components of voltage and current in the driving unit, a modulation factor calculating unit which calculates a modulation factor using the carrier wave data, and a bearing mode changing-over mechanism for effecting change-over from the magnetic bearing mode to the hydrodynamic bearing mode when the calculated modulation factor is outside a predetermined range and for returning to the magnetic bearing mode after the change-over.