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
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