The polarization angle θ1 of a polarizer (14) is set, and the reflection intensity S1 in a cross Nicol state and the reference reflection intensity Ref1 of a liquid crystal cell (15) are measured. A different polarization angle θ2 is then set, and the reflection intensity S2 in a cross Nicol state and the reference reflection intensity Ref2 of the liquid crystal cell are measured. The rations S1/Ref1, S2/Ref2 of measured intensities and the ratio S1·Ref2/S2·Ref1 is determined in order to cancel the background components of the reference reflection intensities Ref1, Ref2 thus determining the value of cell gap accurately.