  A multi-pixel photon sensor with single-photon sensitivity has been
developed, based on a technology of a hybrid photo-detector (HPD) consisting of
a photocathode and a multi-pixel avalanche diode (MP-AD). The developed HPD has
a proximity focused structure, where a photocathode and an MP-AD are facing
each other with a small gap of 2.5 mm. The MP-AD, which has an effective area
of 16x16 mm2 composed of 8x8 pixels, has been specially designed for the HPD.
The gain of the HPD reaches 5x10^4, sufficiently high to detect single photons
with a timing resolution better than 100 ps. Number of photoelectrons up to
four can be clearly identified in a pulse-height spectrum as distinct peaks,
thanks to the low noise characteristics of the HPD. It is also demonstrated
that the HPD can be operated with good performance in a magnetic field as high
as 1.5 T
