A water content sensor for emitting light to a road surface and detecting an amount of water on the road surface based on reflected light reflected by the road surface, the water content sensor comprising: a light emitter for emitting the light toward the road surface, the light including detection light having a first wavelength band in which absorption by water is greater than a predetermined value and reference light having a second wavelength band in which the absorption by water is less than or equal to the predetermined value; a first light receiver for receiving the detection light reflected by the road surface and converts the detection light to a first electric signal; a second light receiver for receiving the reference light reflected by the road surface and converts the reference light to a second electric signal; and a computation processor for detecting the amount of water on the road surface based on a signal ratio between the first electric signal and the second electric signal, wherein the computation processor is arranged to detect the amount of water based on the signal ratio obtained when a signal intensity of at least one of the first electric signal and the second electric signal is within a predetermined range defined relative to a reference value, wherein the water content sensor is characterized in that the computation processor includes a risk table for determining a risk on the road surface based on a relationship between the amount of water and an out-of-range time period that is a period during which the signal intensity of at least one of the first electric signal and the second electric signal is outside the predetermined range, and the computation processor is arranged to calculate the out-of-range time period, and to determine the risk based on the calculated out-of-range time period, the amount of water detected, and the risk table. a light emitter for emitting the light toward the road surface, the light including detection light having a first wavelength band in which absorption by water is greater than a predetermined value and reference light having a second wavelength band in which the absorption by water is less than or equal to the predetermined value; a first light receiver for receiving the detection light reflected by the road surface and converts the detection light to a first electric signal; a second light receiver for receiving the reference light reflected by the road surface and converts the reference light to a second electric signal; and a computation processor for detecting the amount of water on the road surface based on a signal ratio between the first electric signal and the second electric signal, wherein the computation processor is arranged to detect the amount of water based on the signal ratio obtained when a signal intensity of at least one of the first electric signal and the second electric signal is within a predetermined range defined relative to a reference value, wherein the water content sensor is characterized in that the computation processor includes a risk table for determining a risk on the road surface based on a relationship between the amount of water and an out-of-range time period that is a period during which the signal intensity of at least one of the first electric signal and the second electric signal is outside the predetermined range, and the computation processor is arranged to calculate the out-of-range time period, and to determine the risk based on the calculated out-of-range time period, the amount of water detected, and the risk table.