To provide a vertical twin head surface grinding machine which can simultaneously polish both surfaces of an ultra thin workpiece with a high degree of accuracy.

A workpiece conveying device 17 comprises a conveying belt 16 having several holes 35 with which a workpiece W is rotatably held and advancing horizontally, and upper and lower spindles 11, 12 which are adapted to be rotated in one and the same direction and which are arranged in the workpiece conveying direction with a predetermined space therebetween. A distance D between the center axes of both spindles 11, 12 is set so that the polishing surfaces of two grinding wheels 14, 15 overlap with each other as viewed in a plan view, and the width (d) of the overlapping parts is set to be greater than a maximum width of the workpiece but smaller than the radius of the rotary grinding wheels. Further, the conveying belt 16 is arranged so that the widthwise center line C of the belt passes through the center axes O1, O2.