An obstacle determining apparatus includes a distance measuring device, and an obstacle determining portion which determines presence/absence of an obstacle in a measurement space region in front of the distance measuring device. The measurement space region is defined so that a position of a bottom surface of the measurement space region changes stepwisely or continuously according to a depth in a forward direction and/or a width direction of the measurement space region, for example, like a measurement space region D1. The measurement space region may be changed so that the position of the bottom surface of the measurement space region changes stepwisely or continuously according to a detection result of an inclination detecting portion.