To provide a surface treating device for a plate-like workpiece capable of uniformly carrying out surface treatment of both front and back surfaces for a short time.

This surface treating device has a tumbler 1, into which a plurality of plate-like workpieces 2 are input, rotatable around a center axis (a); and a plurality of abrasive grain projecting means 105 for projecting abrasive grains to the plate-like workpieces in the tumbler. The projecting directions of the abrasive grain projecting means are each independently changeable. The plurality of plate-like workpieces 2 are input into the tumbler 1 and the abrasive grains are projected from a plurality of directions A, B while rotating the tumbler. Of the projections, the surface treatment is mainly carried out by the projection of A and the vicinity of a tip end in a rotation direction of a work space V involving the plate-like works 2 in the tumbler is projected by the projection of B. Thereby, inversion of the plate-like workpieces is promoted and a time of the surface treatment is shortened.