A tire-generated sound detection sensor detects a tire-generated sound that is generated from a tire while the vehicle is running. A wheel rotation sensor detects a wheel rotation speed. A preprocessor calculates feature quantities corresponding to a road surface state and tire-generated sound sources. A feature vector generator generates a feature vector having, as components, the feature quantities calculated by the preprocessor and the detected wheel rotation speed. A judgment processor estimates a state of a road surface on which the vehicle is running on the basis of the feature vector generated by the feature vector generator and feature vectors that are stored in a judgment map memory so as to be correlated with a plurality of road surface states, respectively, and each of which corresponds to a road surface state and tire-generated sound sources.