A top-loading-type washing machine including a drum, a drive module for rotating the drum via a drive shaft, inner and outer pulsators placed in the drum to be rotated in opposite directions, and a gearbox connected to the drive shaft for rotating both the pulsators. The gearbox includes a sun gear rotatably connected to the drive shaft, planetary gears rotatably engaged with the sun gear, a ring gear rotatably engaged with the planetary gears, and a carrier for connecting the planetary gears so as to be rotated along with the planetary gears. The washing machine achieves excellent washing performance via rotation of the pulsators in opposite directions and variable speeds of the pulsators relative to the drum depending on the size of the laundry load.