Continuous observations of the vertical ozone profiles between 38 and 76 km in altitude have been made with the millimeter-wave radiometer at Tsukuba, Japan (36.1¿N, 140.1¿E) since October 1996. We analyzed temporal variations of ozone at various altitudes and first identified clearly the features of a semiannual variation between 56 and 68 km. The ozone mixing ratio in this region peaks in January and July, and the amplitude of the semiannual variation at 60 km was estimated to be 13% of the annual average, which is about half the value of 28% observed at 76 km. The phase of the semiannual variation from 56 to 76 km inverts suddenly at around 68 km, suggesting that these semiannual variations of ozone are not induced by the propagation of any kind of wave but are caused by different mechanisms in the regions above and below 68 km.