A new method is proposed to carry out numerical calculations on the shape of field-free core, current distribution, and AC transport losses in self-fields as a function of current amplitude I₀ for high-T_c superconducting tapes with arbitrary cross-sectional geometry. Its validity can be confirmed by verifying that calculated results of AC losses for elliptic tapes agree with the prediction advanced by the theory of Norris. An application to rectangular tapes leads to the finding that the AC losses for them behave as a thin-strip superconductor in high I₀ values near critical current I_c, but gradually deviate from this behavior with decreasing I₀. A decrease in the aspect ratio of width to thickness for rectangles makes the deviation more remarkable. By using this method, the AC transport losses for two kinds of the 7-filamentary tapes with different filament arrangements are calculated, and the results are compared with the experimental behaviors of losses for tapes fabricated by a powder-in-tube method using the Ag sheath. The measured losses can be explained neither by the prediction for an elliptic tape nor by that for a thin strip based on the theory of Norris. Furthermore, the loss values depend on the filament arrangement, which is explained by the calculated results of loss-density distribution. The influence of filament arrangement on the loss behaviors is suppressed as the number of filaments increases to 37, from 7.