of the rotor shaft are automatically adjusted in response to the detected mixing ratio usually the continuous pulping digester may be provided with a low or high pressure feeder consisting of a truncated conical rotary feeder the control apparatus and method of the present invention can be applied to the low or high pressure feeder fig2 and 3 show a front view and a side view of an embodiment of the metal disc of a rotor position detector usable for the present invention respectively referring to fig2 and 3 a metal circular disc 13 having a diameter of for example 1 000 mm is provided with 72 teeth 13 a located in a circular edge portion thereof at angular intervals of 5 degrees the teeth 13a have a height of 10 mm as shown in fig3 two pairs of close relays 14 are arranged in such a manner that the teeth 13a of the metal disc 13 are interposed between each pair of the close relays 14 when the rotor shaft pushing motor 6 revolves and pushes inward or pulls outward the rotor 3 the number of pulse signals output from the proximity switches 14 are integrated by the control device the pushing or pulling distance mm of the rotor is calculated from the integrated value of the pulse signals by the control device after the pushing motor is driven when one pair of proximity switches 14 output a signal 34 on 34 and the other pair of proximity switches 14.