Belle II, as the upgrade of Belle, aims to search for New Physics with 40 times higher luminosity. In order to cope with such a high luminosity, an electromagnetic calorimeter using pure CsI scintillation crystals, whose scintillation time are around 30 nanoseconds, has been proposed. Silicon avalanche photodiodes (APD), making up for the low light yield of pure CsI, are considered as photosensors. The application of the innovative wavelength shifter (WLS) that perfectly matches the emission spectrum of pure CsI and the quantum efficiency of APD maximizes the light collection. The equivalent noise energy of the counter is determined using cosmic rays.