The Japan Meteorological Agency has acquired dissolved oxygen (DO) concentration data each year since 1967 along the 137°E repeat section in the western North Pacific. In this data set we found significant regional temporal trends of decreasing or increasing DO concentrations on various isopycnal surfaces. DO decreases were particularly significant after the mid-1980s in the subtropical gyre; mean rates of DO change at 20–25°N for 1985–2010 were −0.28 ± 0.08 μmol kg−1 yr−1 on 25.5 σθ in North Pacific Subtropical Mode Water (NPSTMW), −0.36 ± 0.08 μmol kg−1 yr−1 on 26.8 σθ in North Pacific Intermediate Water (NPIW), and −0.23 ± 0.04 μmol kg−1 yr−1 on 27.3 σθ in the O2 minimum Layer (OML). The cause of DO decrease differed among isopycnal surfaces. On density surfaces shallower than 26.0 σθ (less than about 400 m), the deepening of isopycnal surfaces and decline of oxygen solubility due to ocean warming have had the greatest influence. In particular, between 25.2 σθ and 25.8 σθ near the NPSTMW their combined contributions accounted for >50% of the DO decrease. In the NPIW core at roughly 26.8 σθ (∼700 m), the decline in DO was attributable to the DO decrease in the formation region. In the OML between 27.0 σθ and 27.3 σθ (∼1000 m), the DO decrease likely resulted from an increase in westward transport of low O2 water due to strengthening of the subtropical gyre. The result of this study shows the importance of the long-term and high-frequency along the 137°E repeat section.