a comparator 31 the comparator 31 compares the data on the calculated roller position with a desired roller position data corresponding to a sampling phase and given thereto by the setting unit 32 and gives a deviation signal representing the deviation of the calculated roller position from the desired roller position to a correction calculating unit 33 the correction calculating unit 33 gives a correction signal corresponding to the deviation at least to the position control unit 23 or the tension control unit 24 referring to fig4 a weaving cycle i e a period in which the main shaft 18 of the loom makes one full turn has a position control period and a tension control period the position control period corresponds to a phase range of for example 270 u00b0 to 20 u00b0 including a beating up moment corresponding to a phase of 0 u00b0 the tension control period corresponds to a phase range of for example 20 u00b0 to 270 u00b0 including a phase angle of for example 180 u00b0 when a maximum shed is formed a position control operation is executed in the position control period to reduce the tension of the satin stripe warp yarns 3 by moving the easing roller 6 forward by the servomotor 15 to a level below a tension set by a tension control operation the tension control operation is executed in the tension control period to increase the tension of the satin stripe warp yarns 3 to a level above the tension set by the position control operation thus.