A radar system of a vehicle has a zero-cross comparator, an integrator, and a sensor. The comparator compares a light reception signal output from each one of photoreceptive elements with a predetermined standard signal and outputs a comparison signal indicating two different states corresponding to a result of the comparison. The integrator samples the comparison signal and converts the comparison signal into one-bit digital data. The integrator integrates the digital data for each photoreceptive element. The sensor compares the integrated data of each photoreceptive element with a predetermined integration standard value. The sensor senses a reflection object based on the integrated data equal to or greater than the integration standard value.