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Photon-number-resolving detectors are in high demand for applications in photonic quantum technology. In this study, we demonstrate the photon-number-resolving capabilities of our self-developed, highly efficient InGaAs/InAlAs single-photon avalanche diode. We achieved intrinsic photon number resolving by harnessing the high multiplication gain generated through an avalanche process in the InAlAs multiplication layer. With a maximum single-photon detection efficiency of 46%, we were able to distinguish photon number states up to 5 from the signal probability distribution without encountering avalanche saturation that could otherwise limit the capability of photon number resolving. We reasonably anticipate that the photon-number-resolving accuracy and capability can be further improved once the noise issue in such InGaAs/InAlAs SPADs is carefully managed.