  The electronic structure of the La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) system has
been studied by angle-resolved photoemission spectroscopy (ARPES). We report on
the evolution of the Fermi surface, the superconducting gap and the band
dispersion around the extended saddle point $k=(\pi,0)$ with hole doping in the
superconducting and metallic phases. As hole concentration $x$ decreases, the
flat band at $(\pi,0)$ moves from above the Fermi level ($E_F$) for $x>0.2$ to
below $E_F$ for $x<0.2$, and is further lowered down to $x=0.05$. From the
leading-edge shift of ARPES spectra, the magnitude of the superconducting gap
around $(\pi,0)$ is found to monotonically increase as $x$ decreases from
$x=0.30$ down to $x=0.05$ even though $T_c$ decreases in the underdoped region,
and the superconducting gap appears to smoothly evolve into the normal-state
gap at $x=0.05$. It is shown that the energy scales characterizing these
low-energy structures have similar doping dependences. For the heavily
overdoped sample ($x=0.30$), the band dispersion and the ARPES spectral
lineshape are analyzed using a simple phenomenological self-energy form, and
the electronic effective mass enhancement factor $m^*/m_b \simeq 2$ has been
found. As the hole concentration decreases, an incoherent component that cannot
be described within the simple self-energy analysis grows intense in the
high-energy tail of the ARPES peak. Some unusual features of the electronic
structure observed for the underdoped region ($x \lesssim 0.10$) are consistent
with the numerical works on the stripe model.
