A development unit performing developing operation for developing electrostatic latent image with toner. The development unit includes a holding chamber wall defining a toner holding chamber, and a development chamber wall defining a development chamber in which a developing roller and a toner supply roller are provided. A partition wall is provided for partitioning the holding chamber from the development chamber. An elongated through hole is formed in the partition wall for bringing the holding chamber into fluid communication with the development chamber. An agitator is rotatably provided in the holding chamber for supplying the toner in the holding chamber into the development chamber through the through hole. A plurality of slats or grids are provided at the through hole, so that a plurality of slits are provided between neighboring slats. These slits allows the toner to pass therethrough from the holding chamber to the development chamber, and these slats restricts return of the toner from the development chamber to the holding chamber.