A control device for coupling with a shaft of a window blind extending in a front-and-rear direction, and for controlling rotation of the shaft in an unwinding direction and a winding direction, said control device comprising: a unidirectional drive unit including a first ratchet seat and a second ratchet seat arranged axially and in the front-and-rear direction and rotatable with each other, said first ratchet seat defining a first surrounding groove which surrounds an axis of the shaft, and having a plurality of first teeth which border said first surrounding groove and which are angularly spaced apart from each other by a plurality of first notches that open toward the winding direction, said second ratchet seat defining a second surrounding groove which surrounds the axis of the shaft, and having a plurality of second teeth which border said second surrounding groove and which are angularly spaced apart from each other by a plurality of second notches that open toward the winding direction; a rotation driving unit including a spool assembly sleeved around said first ratchet seat and having a plurality of first guide slots each extending in the front-and-rear direction and facing radially said first surrounding groove, and a plurality of first rolling balls each slidably engaged in said first surrounding groove and one of said first guide slots such that said spool assembly is operable to rotate in the unwinding direction to drive rotation of said first ratchet seat in the unwinding direction through said first rolling balls, and such that said spool assembly is rotatable in the winding direction to drive movement of said first rolling balls in said first surrounding groove along the winding direction relative to said first ratchet seat so as not to rotate said first ratchet seat; a retaining unit including a tubular case immovably retained, sleeved around said second ratchet seat and having a plurality of second guide slots each extending in the front-and-rear direction and facing radially said second surrounding groove, and a plurality of second rolling balls each slidably engaged in said second surrounding groove and one of said second guide slots such that said second ratchet seat is rotatable with said first ratchet seat in the unwinding direction to drive movement of said second rolling balls in said second surrounding groove along the unwinding direction relative to said second ratchet seat so as to restrict rotation of said second ratchet seat in the winding direction; an output unit disposed forwardly of said retaining unit and adapted to be coupled with and rotate the shaft; and a transmission unit coupled with said output unit and operable to be shifted between a transmitting state where a torque generated as a result of the rotation of said unidirectional drive unit is transmitted to said output unit, and a non-transmitting state where the torque transmission is interrupted. a unidirectional drive unit including a first ratchet seat and a second ratchet seat arranged axially and in the front-and-rear direction and rotatable with each other, said first ratchet seat defining a first surrounding groove which surrounds an axis of the shaft, and having a plurality of first teeth which border said first surrounding groove and which are angularly spaced apart from each other by a plurality of first notches that open toward the winding direction, said second ratchet seat defining a second surrounding groove which surrounds the axis of the shaft, and having a plurality of second teeth which border said second surrounding groove and which are angularly spaced apart from each other by a plurality of second notches that open toward the winding direction; a rotation driving unit including a spool assembly sleeved around said first ratchet seat and having a plurality of first guide slots each extending in the front-and-rear direction and facing radially said first surrounding groove, and a plurality of first rolling balls each slidably engaged in said first surrounding groove and one of said first guide slots such that said spool assembly is operable to rotate in the unwinding direction to drive rotation of said first ratchet seat in the unwinding direction through said first rolling balls, and such that said spool assembly is rotatable in the winding direction to drive movement of said first rolling balls in said first surrounding groove along the winding direction relative to said first ratchet seat so as not to rotate said first ratchet seat; a retaining unit including a tubular case immovably retained, sleeved around said second ratchet seat and having a plurality of second guide slots each extending in the front-and-rear direction and facing radially said second surrounding groove, and a plurality of second rolling balls each slidably engaged in said second surrounding groove and one of said second guide slots such that said second ratchet seat is rotatable with said first ratchet seat in the unwinding direction to drive movement of said second rolling balls in said second surrounding groove along the unwinding direction relative to said second ratchet seat so as to restrict rotation of said second ratchet seat in the winding direction; an output unit disposed forwardly of said retaining unit and adapted to be coupled with and rotate the shaft; and a transmission unit coupled with said output unit and operable to be shifted between a transmitting state where a torque generated as a result of the rotation of said unidirectional drive unit is transmitted to said output unit, and a non-transmitting state where the torque transmission is interrupted.