By wafer holder including a chuck top for mounting a wafer and a supporter supporting the chuck top and having flatness of at most 0.1 mm, a heater unit for a wafer prober and the wafer prober using the wafer holder, a wafer holder and a wafer prober apparatus hardly deformable even under high load and capable of effectively preventing contact failure, and capable of preventing temperature increase in a driving system when a semiconductor wafer having semiconductor chips with minute circuitry that requires high accuracy is heated can be provided. In the wafer holder of the present invention, the flatness of the supporter is preferably at most 0.05 mm, and more preferably at most 0.01 mm.