This study investigates intermediate phonological relationships between /s/(ㅅ) and /z/(ㅿ) in Middle Korean from the 15th century to the 16th century. In Middle Korean, the phonological contrast between /s/ and /z/ was complex. In word-medial intervocalic position, /z/ and /s/ are considered to be contrastive. However, in word-initial or word-final position, they are allophonic, so we can predict which of the two segments will occur. I propose that putting /z/ and /s/ into a gradient and probabilistic model provides insight into the phonological relationship between them. The present paper examines the frequencies of /z/, /s/ and other consonants from the 15th to the 16th century to analyze a historical Korean corpus from the perspective of the Probabilistic Phonological Relationship Model (PPRM), measuring the quantitative analysis index of intermediate phonological relationships. Comparing the entropies between /s/ and other consonants with the entropies between /z/ and other consonants, a continuum defined in terms of entropy reveals that /z/ in Middle Korean was more allophonic than phonemic, and it was lost or merged into /s/ in the 16th century. (Keimyung University)