An actuator used with an optical pickup includes: a base on which a holder
is fixed; a lens holder in which a plurality of installation holes are
formed such that a plurality of objective lenses to record and/or
reproduce optical discs having different recording densities are
installed at different heights; a support member, which movably supports
the lens holder, having one end coupled to the lens holder and the other
end coupled to the holder; and a magnetic circuit which drives the lens
holder in focusing and tracking directions. The plurality of installation
holes include a first installation hole in which a first objective lens
for at least one type of low density optical disc is installed, and a
second installation hole in which a second objective lens for a higher
density optical disc than the low density optical disc is installed. When
a working distance of the first objective lens for the low density
optical disc installed in the first installation hole is represented as
WD1, and a working distance of the second objective lens for the high
density optical disc installed in the second installation hole is
represented as WD2, the first and second installation holes are provided
so that the first and second objective lenses are installed to satisfy
the Equation: WD1>WD2, a separation distance between the optical disc
and the second objective lens=WD2+.alpha., where,
.alpha.=IWD1-WD2|.times.(0.1-1.0). The second installation hole is
located in a more outer circumference of the optical disc than the first
installation hole.