Plural sensors are arranged respectively for plural toner colors, and oscillate to generate and output detection signals with frequencies corresponding to toner densities using oscillator circuits. An analog switch selects one detection signal among the detection signals. A filter decreases amplitude of the selected detection signal. A first signal line transmits the detection signal with the decreased amplitude. A comparator compares the transmitted detection signal with a predetermined threshold value and outputs as the detection signal a signal of which a level is set as a high level or a low level in accordance with the comparison result, and thereby increases amplitude of the detection signal. A receiver-side IC receives the detection signal with the increased amplitude at a predetermined port, determines a frequency of the received detection signal, and determines the toner density on the basis of the determined frequency.