A novel gait phase detection method that can extract the timing of foot contact conditions (Heel Strike and Toe Off) by a single piezo film sensor attached on each leg is proposed. We focused on the occurrence of high-frequency vibrations (> 100 Hz) in the beginning and the end of stance phases during gait. After the features of vibration waveforms during gait were confirmed, we proposed the phase detection method. The optimal parameters were investigated to detect the phases robustly despite the walking speed and the shoe types. Experimental results showed that the detected ground contact events had high accuracy of timing. Finally, we confirmed the feasibility with the prototype wearable device in usual environments.