Hydrogen boding manner of serine (Ser) side chains of sericin, an adhesive protein of Bombyx mori silk fiber, was investigated using 13C solid-state NMR method. The cocoon shell of a fibroin-deficient silkworm strain "Sericin Hope" was used in this study. 13C CP/MAS and DD/MAS spectra of the cocoon shell in dry and wet states were measured. The results showed that the side chains of Ser residues forming β-sheet have low chain mobility in wet state. We assume that these Ser side chains are involved in the formation of hydrogen bonding network via polar zipper interactions.