shown in the preceding figures are designated by like reference characters in fig8 a toner supply tank 61 is arranged above the developing agent tank 2 and formed at its bottom with a neck 61a which extends into the developing agent tank 2 through a hole formed in a cover 62 affixed to an upper portion of the side plate 3 mounted in the neck 61a is a supply roller 63 which is adapted to rotate in the direction of an arrow only when toner particles are supplied from the tank 61 to the developing agent tank 2 a claw 64a formed at a free end of a vibration plate 64 is maintained in pressing engagement with a lower periphery of the supply roller 63 as shown the neck 61a has an open portion at its bottom which is covered with the vibration plate 64 to prevent the falling of the toner particles 24 therethrough the vibration plate 64 is secured at its base by a screw 65 to a bent portion 61b of the neck 61a a member 68 for detecting the toner level is disposed on the upper surface of the toner particles 24 in the tank 61 and connected at its opposite ends to arms secured at their bases to a shaft 69 rotatably connected to the toner supply tank 61 the shaft 69 extends at one end portion thereof out of the tank 61 and supports thereon the detecting disk 41 as shown in fig9 a driving disk 71 which is mounted.