A road surface condition assessing device includes: a tire-mounted device; and a vehicle body system. The tire-mounted device includes: a vibration detector that outputs a detection signal of a vibration on a tire; a waveform processor that generates the road surface data; and a first data communication unit. The vehicle body system includes: a second data communication unit; and a road surface evaluation unit that evaluates the road surface condition. The tire-mounted device transmits an advertise signal including the road surface data indicative of a result of a waveform process on the detection signal and a waveform processing value corresponding to the road surface condition. The vehicle body system evaluates the road surface condition based on the waveform processing value.