A road surface detection device, which detects a road surface of a road on which a host vehicle runs, includes: a sensor configured to detect three-dimensional positional information at individual detection points by scanning a layer that includes the road surface; and an external environment recognition unit configured to calculate two-dimensional positional information in which positional information in a height direction has been removed from the positional information at the individual detection points included in the layer, to calculate, based on the two-dimensional positional information, a point sequence of the individual detection points in a two-dimensional coordinate plane, and to detect a trench existing in the road surface in accordance with a state of change of the point sequence.