A vertical profile of the ion temperature in the ionsphere at local afternoon in the region from 95 to 310 km has been measured directly by means of an improved retarding potential analyzer (RPA) system aboard a space sounding rocket. This rocket was launched at Kagoshima Space Center of Japan (geomagnetic latitude:198¿4'E;longitude:20¿ 0'N). In this paper the method of measurement of the ion temperature by two RPA's and the experimental results are discussed. Two RPA's were flown. One of them measured the probe characteristics and the other one measured the angle of the plasma. To measure the angle of attack, this RPA was tilted at 30¿ from the rocket spin axis. The probe curves were not distorted, as is often the case with rocket probes, since the clean surface of mesh grids and other electrodes were stored in a clean room and the nose cone contained dry N2 gas. The angle of attack is measured directly by the tilted ion trap. The measured angle of attack data agree with the data obtained by geomagnetic and solar aspect sensors aboard the rocket. The tilted probe was most useful in the nighttime experiment when no solar aspect data were available. The results show that the ion temperature below 150 km is the same as that of the neutral temperature at high solar activity. Above 150 km the average ion temperature gradually becomes higher. In this region the ion temperature profile contains a wavelike structure. The peak to trough amplitude of the temperature variation is about 250 ¿K and a vertical wave length of about 25 km is determined. The relationship between this experimental vertical temperature profile and the horizontal atmospheric motions measured by simultaneous ground observations was examined. As a result it was found that these wave data do not fit on the branch of the ϕ-k diagram of theoretical internal atmospheric gravity wave (IGW) by Hines (1960). It may also be difficult to regard another veritcal profiles showing wave lengths of serveral tens of kilometers measured by some other workers as simply manifestations of IGW.