Temporal variation of phosphate concentration in the mixed layer of western subarctic North Pacific from 1968 to 1998 was investigated with quasi‐monthly resolution. February and July phosphate time series showed a linear decreasing trend of −0.008 ± 0.003 and −0.019 ± 0.005 μmol/l/y, respectively, while no multi‐decadal trend was observed in April/May phosphate time series. With this result, net biological phosphate consumption between February and April linearly decreased at the rate of −0.43 ± 0.11 mmolP/m2/y, while that between May and July increased at the rate of +0.16 ± 0.05 mmolP/m2/y. Net biological phosphate consumption from February through August was linearly diminished at the rate of −0.40 ± 0.07 mmolP/m2/y, corresponding to a −0.51 ± 0.09 gC/m2/y decrease of net community production under assumption of the constant C/P ratio of 106. Chl.a inventory in the April mixed layer also showed a decreasing trend of −2.35 ± 1.22 mg/m2/y, which correlated with the observed net production decrease between February and April.