We propose an automatic synchronous PLL system for N-path filter for hum noise reduction in EEG(electroencephalogram) measurement by 0.18 μm CMOS process. Monitored common mode waveform contains Hum noise frequency component during EEG measurement is used for a master clock frequency of the N-path filter. The hum noise frequency as a master clock of the PLL is compared with a rectangular wave obtained by dividing the output frequency of the VCO which is aimed to oscillate at 400 or 480 Hz by 8. The oscillation frequency of the VCO is controlled by the bias voltage. The output signal of the VCO is inputted to a logic circuit that generates an 8-bit digital waveform, and the output for controlling the N-path filter is generated. It is possible to attenuate 50 or 60 Hz component of the EEG signal contains hum noise through an N-path (N = 8) filter. We simulated the PLL (PFD-CP, VCO), logic circuit, N-path filter and measured characteristics of the implemented PLL and logic circuit in this research.