A temperature sensor includes a capacitor having electrodes between which a dielectric body having its dielectric constant variable depending on the variations of ambient temperature is interposed, and a CR oscillating circuit utilizing the capacitor. The dielectric body is made of a ceramic dielectric material which has a working temperature range in the normal dielectric phase with the dielectric constant thereof having a temperature coefficient ranged between -1,000 ppm/.degree.C. and -10,000 ppm/.degree.C. The ceramic dielectric body is also used as a substrate on which the CR oscillating circuit is formed.