A proximity focusing ring imaging Cherenkov detector using aerogel as the radiator has been studied for an upgrade of the Belle detector at the KEK-B-factory. We constructed a prototype Cherenkov counter using a 4 x 4 array of 64-channel flat-panel multi-anode PMTs (Hamamatsu H8500) with a large effective area. The aerogel samples were made with a new technique to obtain a higher transmission length at a high refractive index (n=1.05). Multi-channel PMTs are read-out with analog memory chips. The detector was tested at the KEK-PS pi2 beam line in November, 2002. To evaluate systematically the performance of the detector, tests were carried out with various aerogel samples using pion beams with momenta between 0.5 GeV/c and 4 GeV/c. The typical angular resolution was around 14 mrad, and the average number of detected photoelectrons was around 6. We expect that pions and kaons can be separated at a 4 sigma level at 4 GeV/c.