to use the gravity as the pressure imparting means as in the case of the embodiment of fig2 dispenses with any special arrangement and obviates the necessity of increasing the driving force in each of the embodiments shown in fig1 a and 12 b to 21 the pressure is imparted either constantly or in driving whereas in the case of an embodiment shown in fig2 the driving action is smoothened by imparting pressure only in the event of occurrence of twisting or the like referring to fig2 pitch coils 18p1 and 18p2 are arranged side by side in the yaw direction 11y both the pitch coils 18p1 and 18p2 are provided with slits 112p1 and 112p2 the positions of these pitch coils are detected through these slits by means of light projectors 113p1 and 113p2 and light receivers 114p1 and 114p2 the outputs of the light receivers 114p1 and 114p2 are added together and amplified by an adding amplifier 85 a compensating circuit 86p and a driving circuit 87p form a control driving loop the stationary frame 13 rotates around the optical axis in the direction of arrow 88 upon occurrence of a twist this rotation is detected by a differential amplifier 89p in response to the output of the amplifier 89p electromagnets 91p1 and 91p2 are driven by an electromagnet driving circuit 99p magnetic targets 91p1 and 91p2 which are mounted on the boss.