A unified expression for the probability distribution function with quantized state variables for a stochastic noise environmental system is derived in the form of a statistical orthogonal expansion. Next, a new statistical procedure for estimating the precise noise level probability distribution form on the basis of the roughly observed data with a quantized level of a large‐difference interval is proposed as an application of the above unified theory. The validity of the method has been experimentally confirmed by applying it to actual indoor noise data observed in a reverberant room of our university, and to outdoor noise data observed in Hiroshima City itself.