The main component of seashells is calcium carbonate (CaCO3). When seashells are calcined at high temperatures, CaCO3 becomes calcium oxide (CaO), and this CaO exhibits antimicrobial activity. In this study, we attempted to develop a transparent coating that retains antimicrobial activity for a long time by mixing polyvinyl butyral (PVB), which has excellent adhesive and binding properties, with heated shell powder (HSP). The scallop HSPs used in this study were nanoparticles with a particle diameter of approximately several hundred nm, and the prepared paint showed high transparency. Elemental analysis showed that scallop HSP particles existed in the paint as Ca(OH)2. The antimicrobial activity of the surface applied with scallop HSP-containing PVB (HSSP-PVB) paint was then evaluated using JIS Z 2801 and ISO 21702: 2019. The HSSP-PVB paint-applied surfaces showed high antibacterial and antiviral activity. The antimicrobial activity of the scallop HSP-PVB paint-applied surface was attributed to the creation of a strongly alkaline environment due to surface hydration, and the strongly alkaline environment was maintained for a long period of time. It was suggested that the PVB covered the surface of the scalloped HSP particles, which significantly prevented the HSP from contacting CO2 and H2O molecules in the air.