Summary: The existence of thing-in-itself has been proposed and maintained its obscurity by Immanuel Kant ever since 1770. Listing 7 features of the thing-in-itself of Kant's, we find that the wave function ψ of Quantum Mechanics of modern physics possesses all those 7 features and that it is explicitly calculable in mathematical form. Furthermore，in Quantum Mechanics the noumenal ψ，its physical property p，and the corresponding mathematical operator for the physical property p are distinctly defined and expressly related by and equation. We also give detail explanations why a complex wave function cannot be observed. Our conclusion is that we propose to have found a concrete and distinct mathematical form for the thing-in-itself in the Quantum Mechanics regime，which may serve as the non-sensible noumenal cause for an appearance of a body. As the thing-in-itself is non-sensible and non-observable，it appears that only mathematical form of thing-in-itself is proper and accessible. The philosophical obscurity or mystery on noumenon of an object existing for hundreds of years appears now resolved.
