The vertical movement of a chum salmon Oncorhynchus keta returning to Japan was examined using a depth-recording archival tag in the western North Pacific Ocean over a 23-day period. The chum salmon showed clear diel vertical movements, with an average swimming depth of 12.8 m during the day and 4.8 m at night. The chum salmon spent 48% of the day and 85% of the night in the upper 10 m, 30% of the day and 10% of the night in the 10–20 m layer, and 20% of the day and 5% of the night in the 20–60 m depth layer. The chum salmon remained in the top part of the water column and rarely went below 40 m and only went below 50 m on 3 days out of the 23 days. The diel vertical movement of the chum salmon may be related to maximization of foraging efficiency.