A laser printer (10) uses an elongated toner cartridge (24) adapted for insertion into a toner cartridge slot (25) defined within a body housing (12) of the printer (10). The slot (25) is elongated in shape and extends horizontally. The toner cartridge (24) extends along the slot (25) when received in it. The slot (25) has a mouth at one longitudinal end through which the toner cartridge (24) is inserted in and removed from the slot (25). A slot door (26) is provided for the mouth of the slot. The slot door (26) is supported to be movable between a first position for substantially closing the mouth and a second position for leaving the mouth open. The slot door (26) has inner and outer sides, the inner side facing toward inside of the slot (25) when the slot door (26) is located at the first position while facing upward when the slot door (26) is located at the second position. The slot door (26) has guide surfaces formed on its inner side for guiding the toner cartridge to establish alignment of the toner cartridge (24) relative to the slot (25) preparatory to inserting the former into the latter. The slot door (26) includes a toner collector vessel (62) for collecting any toner falling down from a used toner cartridge during its removal from the slot (25). The slot door (26) facilitates toner cartridge replacement, prevents ambient light from entering the slot (25) and avoids contamination of environment of the printer (10) with toner.