  We present a novel hydrated layered manganate MgMn$_3$O$_7$$\cdot$3H$_2$O as
a maple-leaf-lattice (MLL) antiferromagnet candidate. The MLL is obtained by
regularly depleting 1/7 of the lattice points from a triangular lattice so that
the magnetic connectivity $z = 5$ and is thus intermediately frustrated between
the triangular ($z = 6$) and kagom\'e ($z = 4$) lattices. In
MgMn$_3$O$_7$$\cdot$3H$_2$O, the Mn$^{4+}$ ions, carrying Heisenberg spin 3/2,
form a regular MLL lattice in the quasi-two-dimensional structure.
Magnetization and heat capacity measurements using a hydrothermally-prepared
powder sample reveal successive antiferromagnetic transitions at 5 and 15 K. A
high-field magnetization curve up to 60 T at 1.3 K exhibits a multi-step
plateau-like anomaly. We discuss the unique frustration of the MLL
antiferromagnet in which the chiraldegree of freedom may play an important
role.
