A toner reservoir for a laser printer includes an elongated toner hopper housing extending in a horizontal direction and an elongated toner cartridge adapted for detachable attachment on the toner hopper housing with its bottom on the top of the latter. The toner hopper housing has an elongated top opening extending longitudinally of the toner hopper housing. The toner cartridge has an elongated bottom opening extending longitudinally of the toner cartridge. The top and bottom openings are adapted to mate with each other when the toner cartridge is attached on the toner hopper housing so as to allow discharge of toner through them from the toner cartridge into the toner hopper housing. The bottom opening is closed by a strip of sealing tape, which is removed away after the toner cartridge is attached on the toner hopper housing. A toner sensor is provided in the toner hopper housing. The toner hopper housing has upright partition walls dividing the space in the toner hopper housing into local regions arranged along the length thereof, such that any toner falling down from the toner cartridge into the toner hopper housing upon removal of the sealing tape is substantially prevented by the partition walls from moving from one of the local regions into another. In this manner, the toner level in the toner reservoir is effectively equalized along its length upon toner replenishment, allowing appropriate toner level detection by a single toner sensor.