  The electronic structure of ThPd$_2$Al$_3$, which is isostructural to the
heavy fermion superconductor UPd$_2$Al$_3$, was investigated by photoelectron
spectroscopy. The band structure and Fermi surfaces of ThPd$_2$Al$_3$ were
obtained by angle-resolved photoelectron spectroscopy (ARPES), and the results
were well-explained by the band-structure calculation based on the local
density approximation. The comparison between the ARPES spectra and the
band-structure calculation suggests that the Fermi surface of ThPd$_2$Al$_3$
mainly consists of the Al $3p$ and Th $6d$ states with a minor contribution
from the Pd $4d$ states. The comparison of the band structures between
ThPd$_2$Al$_3$ and UPd$_2$Al$_3$ argues that the U $5f$ states form Fermi
surfaces in UPd$_2$Al$_3$ through hybridization with the Al $3p$ state in the
Al layer, suggesting that the Fermi surface of UPd$_2$Al$_3$ has a strong
three-dimensional nature.
