  We have developed a sensor system based on an optical photon-counting imager
with high timing resolution, aiming for highly time-variable astronomical
phenomena. The detector is a monolithic Geiger-mode avalanche photodiode array
customized in a Multi-Pixel Photon Counter with a response time on the order of
nanoseconds. This paper evaluates the basic performance of the sensor and
confirms the gain linearity, uniformity, and low dark count. We demonstrate the
system's ability to detect the period of a flashing LED, using a data
acquisition system developed to obtain the light curve with a time bin of 100
microseconds. The Crab pulsar was observed using a 35-cm telescope without
cooling, and the equipment detected optical pulses with a period consistent
with the data from the radio ephemeris. Although improvements to the system
will be necessary for more reliability, the system has been proven to be a
promising device for exploring the time-domain optical astronomy.
