We calculate the intensity of far‐infrared absorption of a single electron and two electrons in a square‐well quantum dot under an external electric field. The intensity is effectively modulated by an applied electric field in different ways, according to the number of electrons in the quantum dot. The difference is due to the effect of Coulomb interaction between electrons. The Stark effect on optical transition of interacting electrons confined in a quantum dot is discussed. When a quantum dot is selected out of an array of quantum dots by a local electric field, we can detect the number of electrons in it using electric‐field‐induced far‐infrared absorption spectroscopy. ©1995 American Institute of Physics.