1. A road surface condition-detecting system for a vehicle having wheels, which detects a condition of a road surface on which said vehicle is traveling, comprising: road noise-detecting means arranged in the vicinity of at least one of said wheels of said vehicle for detecting road noise generated from said at least one of said wheels; means for extracting data of a parameter of a plurality of frequency components of said road noise, from said road noise detected by said road noise-detecting means; and determining means for determining said condition of said road surface on which said vehicle is traveling, based on said data of said parameter of said plurality of frequency components of said road noise, by means of a neural network.