PURPOSE: To detect a bubble if a fluid while discriminating from other pressure disturbance by filtering the pressure detection signal of a pressurized liquid thereby removing two signals lower than a predetermined reference level, a short duration signal and a long duration signal.

CONSTITUTION: A pressure detection means 58 detects pressure drop of a pressurized fluid delivered through a nozzle 56 continuously to generate a signal 60 corresponding to the pressure varying with time. The signal 64 is amplified 61 and a a negligible waveform having extremely small amplitude is removed through an amplitude filter 62 thus delivering a signal 64 to a band-pass filter(BPF) 66. The BPF 66 compares positive and negative reference levels being set in a high-pass filter 66A and a low-pass filter 66B with the time interval between the intersection of respective waveforms and removes a short duration waveform or signal of abrupt variation not corresponding to the waveform of bubble and a long duration waveform or signal varying slowly through filtering. An AND 66C delivers a signal 68 indicative of passage of the signal 64 through both filters thus indicating that a bubble is generated at the nozzle 56 or passed through the nozzle 56 by a bubble designation means 70.