In many locations worldwide, ostreid herpesvirus 1 (OsHV-1) infections are associated with mortalities in Pacific oyster Crassostrea gigas primarily during the summer months. In the present study, experimental infections were performed to investigate the pathogenicity of OsHV-1 JPType1 (a Japanese variant) in hatchery-reared Pacific oyster larvae and spats. The JPType1 virus was prepared from dead oyster spats obtained from a sea cage and was once passed in oyster larvae as the inoculum source of the infection experiments. Compared to pediveliger larvae, D-shaped larvae were more susceptible to infections when the larvae were exposed to serially diluted virus solutions at the concentrations of 3.2 × 106, 3.2 × 105 and 3.2 × 104 virus DNA copies/mL. Real-time PCR analysis revealed 1.6 × 108 virus DNA copies/ng total DNA in a pooled sample of D-shaped larvae. No mortality was observed after 5 days when nine spat groups of differently sizes (mean shell height from 1.8 mm to 6.1 mm) were immersed in seawater containing 2.5 × 106 virus DNA copies/mL; however, 6.3 × 105 virus DNA copies/mg whole weight were detected in a group of spats. Virus concentrations were higher in smaller spat groups than in larger spat groups. The pathogenicity of OsHV-1 JPType1 in larvae and spats was confirmed; however, it declined with the growth of the oyster.