The Fukushima Daiichi nuclear power plant experienced multiple meltdown events after losing all power after an earthquake and its related tsunami disaster in 2011. Since the Fukushima Daiichi nuclear power plant still has regions of intense radiation, robots with a high radiation tolerance are necessary to complete work in decommissioning the reactors. Such robots frequently require proximity sensors. We have chosen an ultrasound sensor as a radiation-hardened sensor because ultrasound sensors consist of piezoelectric ceramic, which is very robust against radiation. However, although the ultrasound sensor itself is robust against radiation, since a semiconductor receiver circuit must be constructed to amplify the signal received from the ultrasound sensor, we have designed a radiation-hardened receiver circuit and have constructed an ultrasonic sensor system. This paper reports evaluation results demonstrating that the radiation-hardened ultrasonic sensor system has at least a 94 Mrad total-ionizing-dose tolerance.