We propose a power management method using a digital voice activity detection (VAD) module for intelligent ubiquitous sensor systems. Only when this VAD module detects a speech signal, a main signal processing circuit is connected to a power source. While a speech signal is undetected, most of circuits except VAD are blocked off. This leads effective stand-by power reduction for the sensor node specialized in the speech signal processing. We implemented the VAD algorithm, utilized the zero crossing of input signals, to an FPGA. The FPGA implementation achieves 2.10mW operation. Furthermore we synthesize this VAD module using CMOS 0.18-μm process, and achieve 3.49μW power consumption at the supply voltage of 1.8 V and at 100 kHz operation.