A proximity focusing ring imaging Cherenkov detector using aerogel as the radiator has been studied for upgrading the Belle detector at the KEK-B-factory. We constructed a prototype Cherenkov counter using a 4×4 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 π2 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 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 σ level at 4 GeV/c .