Measurements of O₃, NO_x and SO₂ concentrations were performedat Bijodaira (altitude 977m), on the westernn slope of Mt. Tateyama, in the summer and autumn of 2004 and 2005. A clear diurnal variation of O₃, reaching amaximum during the afternoon, was observed in the summer and early autumn. No clear diurnal variation was seen in November. However, NO _x concentration was higher in the daytime than in the nighttime both in the summer and in the autumn. The difference in diurnal variation of O₃ may be due to the photochemical formation of O₃ during the summer and autumn. High concentrations of SO ₂ were sometimes observed in November. The backward trajectory analysis showed that the high concentrations of SO₂ at Bijodaira originated from the pollutant regions of the Asian continent.