© 2020 IEICE. Temperature monitoring is indispensable for realizing a long life in ICs, since reliability of ICs is affected by the temperature. We propose a temperature sensor based on an oscillation frequency of a relaxation oscillator embedded in an IC. In order to examine feasibility of the temperature monitoring, we designed a sensor circuit utilizing a relaxation oscillator and examined it by SPICE simulations. The simulation results show that temperatures in ICs can be estimated by a single regression straight line with a small error under process variations of MOSs in the ICs.