A shredder comprises a cutter unit including a pair of rotary cutters for shredding a material therebetween while the rotary cutters are rotating in a normal direction, a reversible motor for rotating the rotary cutters through a speed reducer, and a switch unit for selectively causing the motor to rotate the rotary cutter in the normal direction or reversing the motor to rotate the rotary cutters in a reverse direction to remove the material from the rotary cutters when the material is jammed in the rotary cutters. An electrical control circuit is responsive to actuation of the switch unit for enabling the motor to produce a larger torque while rotating in the reverse direction than the torque produced while rotating in the normal direction, thereby reliably releasing the rotary cutters from biting engagement with the material when jammed. The switch unit has means for generating a time delay while switching between rotation of the motor in the normal direction and rotation of the motor in the reverse direction, so that the motor and the speed reducer are protected against an undue load when starting to rotate in the reverse direction.