  The electrical, magnetic, and structural properties of
Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$ (0 $\leq x \leq$ 0.2) are investigated. The
parent compound Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal, critically close to
magnetic order. We have found that, with a Ru-site doping by only a few percent
of Mn, the ground state is switched from a paramagnetic metal to an
antiferromagnetic insulator. Optical conductivity measurements show the opening
of a gap as large as 0.1 eV, indicating that the metal-to-insulator transition
is driven by the electron correlation. The complex low-temperature
antiferromagnetic spin arrangement, reminiscent of those observed in some
nickelates and manganites, suggests a long range orbital order.
