Ground based measurements of ozone as well as carbon monoxide have been carried out at Happo station. Annual cycles of ozone and CO concentration were obtained. Monthly means of ozone show maximum of ca. 65 ppbv in April or May and minimum of ca. 45 ppbv in December or January. The annual behaviour of CO is somewhat similar to that of ozone showing maximum of 225 ppbv in April and minimum of 151 ppbv in December. A positive correlation was obtained between ozone and CO during spring and summer. This fact indicates that photochemical ozone production in the troposphere is active in spring and summer. It should be noted that the highest correlation was obtained in April and May which corresponds to the month giving the ozone maximum concentration. The comparison between ozone and humidity in late spring revealed that although many events showing dry air parcels reaching the site were observed, simultaneous increase of ozone was not observed at all. This fact clearly indicates that the sporadic high concentration of ozone in spring is caused by the photochemical ozone formation in the troposphere rather than the intrusion of stratospheric air. ¿ 1998 American Geophysical Union