In a rain sensing device, a sensing device that can estimate the size of raindrops or the like without a large amount of hardware resources, and its sensing method, and a wiper controlling apparatus using the same can be provided. In a sensing device, light emitted from a light-emitting member is introduced to a transparent plate, reflected by a sensing surface of the transparent plate and then received by a photo-detector, thereby detecting a condition of the sensing surface. The sensing device includes a sampling part for sampling signals from the photo-detector, a compression part for compressing signals in a sequence of the sampled signals when they continue within a single segment, where a plurality of the segments defined by a plurality of predetermined values are set with respect to an input signal, a calculation part for calculating a compressibility from the number of the sampled signals to be compressed by the compression part and the number of the sampled signals that have been compressed thereby, and a judging part for judging the condition of the sensing surface based on the compressibility.