The aim of this study was to investigate the effect of the time of day on selectry immunogloblin A (sIgA). Six males participated in this study. Saliva samples were collected for each subject at 06:00, 06:30, 07:00, 11:30, 18:00, 22:30 and 06:00 next morning on both routine diet day and prescribed doet day. SIgA concentration was determined by an enzyme linked immunosorbent assay, and total saliva protein concentration was determined by BCA protein kit. There was no significant diurnal variation for salva flow rate in each day. SIgA concentration and secretion rate were high in the morning and low in the afternoon and evening on both days, and the levels of sIgA concentration at 18:00 and 22:30 were significantly lower than that at 06:00 next morning on the prescribed diet day. Both sIgA concentration and secretion rate had no significant difference between routine diet day and subscribed diet day. The levels of total saliva protein concentration and secretionrate at 18:00 were higher in routine diet day than in prescribed diet day. These results suggested that sIgA concentration showed diurnal variation on prescribed diet day.