From external electric field dependence of photocurrent spectra of a quantum wellp‐i‐nphotodiode connected to a series resistive load, it is predicted and directly demonstrated that photocurrent bistability is obtained with respect to the variation of the external bias voltage when the device is illuminated by an incident ray in a certain wavelength region. Furthermore, additional self‐consistent interpretation is given with empirical data taken for the system with no series resistor.