The preparation of a thick-film magnetic semiconductor (TMS) using ferrite powder and a ruthenium compound is described, and the application of its properties to a multipurpose sensor is considered. The TMS was sintered at a relatively low temperature and showed ferromagnetic and semiconducting properties which were dependent on the temperature, humidity, and gas concentration. The TMS is a versatile device: a temperature-sensitive magnetic core, a thermistor, and a varistor can be added to ordinary resistance and capacity elements. Considering these characteristics, the TMS can be used as a temperature/humidity/gas sensor for a modulated environmental monitor. A novel temperature-signal readout method due to the magnetic head is also introduced for the TMS pattern sensor. >