(EN) ABSTRACT Provided is a film thickness measurement device, including: a head that holds a substrate to be polished and is capable of moving the substrate, a stage made of transparent material; a liquid supply unit configured to supply liquid onto the stage; a liquid discharge unit configured to discharge the liquid on the stage to the outside; a measurement unit configured to be placed on a side opposite to the head across the stage and to optically measure a film thickness on a surface of the substrate that is placed across the stage; and a control unit configured to move at least one of the stage and the head toward the other, and to irradiate the substrate with light while the surface of the substrate is immersed in the liquid, thereby performing film thickness measurement. Fig. 8 34