To provide a road surface state-detecting sensor which can detect a state of a road surface in a wide range in a simple constitution.

When an optical fiber 20 laid along a road surface is heated for a predetermined time with a predetermined calory, a temperature change width ΔT as temperature information obtained at the optical fiber 20 shows a different value depending on a wetness of the road surface. A heat time constant τ also varies depending on the wetness of the road surface. Therefore, when a heater 22 arranged in the vicinity of the optical fiber 20 is intermittently heated and a temperature distribution of the optical fiber 20 is measured, the wetness of the road surface can be detected. The wetness of the road surface is obtained from the temperature change width ΔT and heat time constant τof the optical fiber 20 in the vicinity of the heater 22. The presence or absence of freezing of the road face is detected by measuring a temperature value of a position of the optical fiber 20 away from the heater 22.