To provide a measuring apparatus for the concentration of a developer by which the concentration of the developer can be measured with a simple constitution and with high accuracy.

A concentration measuring part 4 which measures the concentration, as the toner amount in a solvent, of a developer is installed, in a position in which a circulation direction is not vertical, in a supply pipe line 6 by which the developer is supplied to the outer circumferential face of a photoconductor belt. A pair of glass rods 9 opposite end parts of which protrude into the supply pipe line 6 are installed at the concentration measuring part 4. A light emitting part in which a light emitting diode 11 used to irradiate infrared rays is arranged and installed is installed at the other end of the glass rod 9 on one side. A light receiving part 8 in which a light receiving diode 12 used to receive the infrared rays is arranged and installed is installed at the other end of the glass rod 9 on the other side. The influence of the sediment of a toner is excluded, the glass rods 9 clean the developer, and the concentration of the developer can be measured with high accuracy. By using the infrared rays, the influence of a color mixture is eliminated even with reference to a color, and the concentration of the developer can be measured with high accuracy. A GAP tolerance is large, and the production property of this measuring apparatus can be enhanced.