In a precessional operation where an X-ray irradiation means and a two-dimensional radiation detector are moving on a circular or elliptic orbit respectively by rotating either one of holding means, that is a C arm or a holding means at angular rate of a·sin(t) under a steady state condition and by rotating the other holding means at angular rate of b·cos(t), the operation is carried out at angular rate (f(t), g(t)) to ensure that the maximum acceleration is smaller than the steady state acceleration during a running-up period until steady is reached.