In the 5th Volcanic Eruption Prediction Plan in Japan, the seismic prospecting at volcanoes has been promoted in order to understand the relation between the structure of volcanoes and their volcanic process. Although many volcanologists have realized the significance of the seismic prospecting at volcanoes, they could not solve difficulties which would be encountered in trying to accomplish it. One of the difficulties is due to the lack of portable recorders with a precise timing system. Therefore, we developed the small-size, light-weight and high-performance data logger to overcome the above problem and to realize the seismic observation at volcanoes. The new data logger named “LS8000SH” is characterized as follows : 1) The timing is corrected automatically by a built-in GPS receiver, and its accuracy is always kept within 1 mili-second. Based on this data logger, we have realized the “cableless” multichannel data acquisition system for seismic prospecting. 2) The power consumption is normally 330 mW during data acquisition. The minimum one is less than 5 mW during a standing-by and waiting for recording. It is small enough for outdoor operation. 3) The dimension of the logger is 258 mm × 148 mm × 40 mm, and its weight is about 950 g. The small-size and light-weight body enhances much portability at volcanoes. 4) The number of input channels is 4. The gain-variable amplifier and the AD converter with 16 bits resolution are equipped. The maximum sampling rate is 1000 Hz. The data are stored in NAND type flash memory (maximum 20 MB). 5) The firmware written in C language is stored in the flash memory and can be reformed and replaced freely. Parameters such as a sampling rate, a cutoff frequency of the filter, an amplifier gain, a recording time and so on are easily set through IBM/PC and its compatibles. The data recorded and stored in the flash memory are also retrieved with the speed of 200 KB/s through PC’s bi-directional printer port. We performed seismic explosion experiment at Kirishima Volcano in the southern Kyushu, Japan in the period from Nov. 26 to Dec. 2, 1994. We deployed 163 data loggers in and around the volcano and recorded seismic signals from six explosive sources. We could obtain data with well-calibrated time information and good SN ratio from more than 95 percents of the installed loggers. Only several ones did not work correctly mainly due to operational problems such as GPS antenna. The high qualities of the data proved that the newly developed data logger was a good recorder for the seismic prospecting at volcanoes.