Microsensors have a potential to drastically improve subsurface measurement, because they have advantages in cost, installation, mobility and sensitivity to the conventional sensors. The authors have been developping a silicon capacitive micro accelerometer for downhole seismic detector under "Subsurface Microsensing Project" in Tohoku University, Japan. In this paper, the concept, design, packaging and field test of this seismic detector with microsensor is described. The micro accelerometer is fabricated using silicon and glass, and has a size of 9mm×5mm×2.2mm. The sensor has comparable sensitivity and frequency characteristics to high sensitive piezoelectric accelerometers. Three sensors are mounted on a circuit board and assembled in a sonde which protects the sensor/circuit from water in borehole. In situ detectability of seismic wave was evaluated in a small test field using artificial seismic source.