vertical movement of the pushrod 107 and thus the resonant frequency of the lc resonant circuit 122 is altered in the above described configuration of the present embodiment the weight of the solution container 3 with the developing solution therein is proportional to the resonant frequency of the current output from the lc resonant circuit 122 that is the weight of the solution container 3 including the developing solution can be obtained from the resonant frequency of the current from the lc resonant circuit 122 as described above the toner density of the developing solution in the solution container 3 is maintained to be substantially constant therefore with a specific gravity corresponding to the density of the developing solution and the weight of the developing solution in the solution container 3 obtained by subtracting weight of the solution container 3 from the total weight of the solution container 3 including the developing solution the amount i e a volume of the developing solution in the solution container 3 can be detected it should be noted that the weight sensor unit is not limited to a sensor unit with an lc resonant circuit as described above but may be any weight sensor unit for example using a strain gauge as long as the weight sensor unit is capable of holding and measuring weight of the solution container 3 it should be noted that in the measuring unit with the weight sensor unit 105 as described above the weight sensor unit 105 is prevented.