Head-related transfer functions (HRTFs) play a crucial role in sound localization by human auditory system. Many researchers have investigated how HRTFs relate to sound localization, and have developed virtual auditory displays (VADs) which present virtual sound sources synthesized with HRTF-based audio signal processing. However, HRTFs have strong individuality because each listener's head and external ear shape differ. Therefore, an ideal 3D auditory space synthesis using HRTFs necessitates personal HRTF measurements or simulations, thereby degrading the versatility of such HRTF-based VADs. Resolution of problems related to HRTF individual variation necessitates clarification of a physical mechanism that yields spectral notches and peaks depending on a pinna shape. Researchers have therefore investigated the relation between HRTFs and the pinna shape using measurements of real and artificial heads or ear replicas. Moreover, numerical simulations, such as the boundary element method (BEM) or the finite difference time domain (FDTD) method, are considered as practical means to study this issue. As described herein, surface pressures on a pinna and HRTFs are calculated using BEM for various sound source elevation angles. The simulated surface pressures are analyzed in the time and frequency domains. Each boundary element on the surface is regarded as a secondary source radiating a sound wave corresponding to a reflection from the surface region, thereby enabling direct observation of a pinna's effects on HRTFs. Numerical results demonstrate the extent to which each part of a pinna contributes to a production of HRTF spectral notches and peaks depending on a source elevation.