An electrostatic latent image-developing device includes a toner cartridge provided with a container having a toner discharge opening, a toner contained in the container and a sealing tape for sealing the toner discharge opening, and a toner cartridge-holding device having a toner-receiving opening and allowing detachable mounting of the toner cartridge. The toner cartridge holding device has a toner cartridge mount detection switch that is opened and closed by the attachment or detachment of the toner cartridge. The toner cartridge mount detection switch is inserted in an electric circuit that feeds electric power to an electric motor that drives a conveying device for conveying the discharged toner from the cartridge to the developing unit or drives a take-up shaft for taking up the sealing tape.