A three-dimensional boiling water reactor (BWR) core simulation program, COSMO-2, has been developed on the basis of a few-group coarse-mesh diffusion scheme with one mesh per assembly, in which the accuracy is improved by use of effective diffusion coefficients to accurately evaluate the net neutron current at the interface between neighboring assemblies. As an experimental verification of the COSMO-2 model, the first-cycle operation of a typical BWR is simulated, and the accuracy of the simulation is evaluated quantitatively in terms of standard deviation from the measured whole-core traveling in-core probe (TIP) data. To calculate the TIP reading from fuel assembly power obtained from COSMO-2, the relation between TIP reading and average power of fuel assemblies is generated by a three-dimensional local core analysis program, FASMO. Good agreement is obtained between measurement and calculation. The maximum value of the root-mean-square (rms) error is 6.3%, including the asymmetric nature of measured data and the measurement uncertainty (3% in rms).