Latent electrostatic image developing device

In a latent electrostatic image developing device for developing a latent electrostatic image formed on an image-bearing member by applying a developer thereto, said developing device comprising developer applicator means for holding developer on the surface thereof, carrying the developer to a developing zone and applying it to a latent electrostatic image in the developing zone and bias voltage applying means for applying a bias voltage to the developer held on the surface of the developer applicator means;

• the improvement wherein the bias voltage applying means comprises a first dc fixed voltage supply and a second dc fixed voltage supply and wherein the developing device further includes detector means for detecting the relative humidity of the environmental atmosphere and control means for connecting both the first and the second dc fixed voltage supplies to the developer held on the developer applicator means to adjust the applied bias voltage to a first value when the detected relative humidity is less than a predetermined value, and connecting only the first dc fixed voltage supply to the developer held on the developer applicator means to adjust the applied bias voltage to a second value less than the first value when the detected relative humidity exceeds the predetermined value.