Cognitive functions like learning and memory depend on the time of day in various organisms. We have demonstrated that long-term recognition memory performance revealed an apparent diurnal/circadian change with the peak during the early night in nocturnal mice. SCOP (PHLPP1b) is a key molecule regulating hippocampus-dependent long-term memory in mice. The amounts of SCOP in the hippocampus exhibit robust circadian changes. Long-term memory formation in recognition memory is regulated by the circadian clock through SCOP dynamics in the hippocampal CA1 in nocturnal mice. Whether diurnal changes and SCOP-dependent regulation of memory performance are conserved across species with diurnal/nocturnal habits remains unknown. We used diurnal Japanese macaques to see how the time of day affects memory performance. Also, we tested whether SCOP-dependent regulation of memory performance is conserved across species. The memory association between the bitter taste of drinking water and the nozzle color of the water bottle was established to evaluate memory performance for macaques. Using this method, we found the diurnal variation of declarative memory in Japanese macaques. The middle of the daytime is the most effective time for memory performance during the light period. We interfered with SCOP expression by using shRNA expressing lentiviral vector. Scop knockdown in the hippocampus abrogated the memory performance in the middle of the daytime. Our results suggest that SCOP in the hippocampus is necessary for the diurnal rhythm of the memory system and that the SCOP-dependent memory regulation system could be conserved in mammals.