There are provided an inter-vehicle distance detecting device and an inter-vehicle distance detecting method which can precisely measure an inter-vehicle distance from a user's own vehicle to a neighboring vehicle even if a radar becomes unable to detect the neighboring vehicle due to the influence of multipath or the like. If the radar does detect a neighboring vehicle, a width of the neighboring vehicle is calculated based on a precise inter-vehicle distance determined by the radar and a precise vehicle visual angle determined from an image taken by a camera, and is stored in a memory, and if the radar becomes unable to detect the neighboring vehicle, an inter-vehicle distance to the neighboring vehicle is calculated according to a trigonometric function based on the precise width of the neighboring vehicle previously stored in the memory and a precise vehicle visual angle determined from an image taken by the camera at this time.