An excrement sensing device and a diaper with an excrement sensing device are provided. The diaper includes a detachable sensing part, an electronic part and a diaper main body. The detachable sensing part includes a first light guide structure and a second light guide structure. The electronic part includes a signal processor. A light guide gap is formed between two light guide openings of the first light guide structure and the second light guide structure. After the optical signal from the emitter is transmitted through the first light guide structure, the light guide gap and the second light guide structure, the optical signal is received by the receiver. When excrement enters the light guide gap, the optical signal is subjected to a change. In response to the change of the optical signal, the signal processor generates a warning signal.