In the binaural method, sound signals are reproduced in each of the ears of a listener by using headphones. Usually, a dummy head microphone is used to record the signal to be reproduced. However, if the sound field to be reproduced does not exist, the dummy head recording method is not usable. In this case, we can produce the virtual sound field by calculating the input signals of the headphones from the head-related transfer functions (HRTFs) and the virtual sound image. Although the HRTFs can be obtained by measuring an impulse response between a loudspeaker and a listener's ear in an anechoic chamber, it would be very useful if they could be calculated. In many researches that focus on predicting HRTFs by calculation, the boundary element method (BEM), which calculates the HRTFs by solving the wave equation using a computer model of the head, is rather prominent. However, one of the drawbacks of the BEM in obtaining the HRTFs is the considerable amount of calculation time that is required. In this study, we suggest a new method for HRTF calculation which enables significantly enhanced speed by pre-processing an inverse of the coefficient matrix of the BEM.