A new type of magnetic field optical sensor with a diode-pumped solid-state microchip laser was studied. It was found that use of the self-mixing laser intensity modulation method combined with an acousto-optic frequency shift enabled the detection of Faraday signals in an extremely weak feedback field. Through experiments using an Nd:YVO₄ microchip laser and a YIG crystal as a Faraday element, modulation signals proportional to the applied magnetic field were obtained in the very weak amplitude feedback region of 10^-6.