Providing a slider for a concealed slide fastener, capable of achieving a smooth sliding operation of the slider, even when the slider is applied to a seat cover for a seat and strong lateral pulling force is applied to the slider when the slide fastener is closed. A slider for a concealed slide fastener comprises a slider body having at least a lower blade, left and right side wall portions and left and right first flanges, and slider upper plate member engaged with the slider body and having at least a second flange projecting toward at least the left and right side wall portions.