  Motivated by a recent finding of a spin-flop phenomenon in a system without
anisotropy in spin space reported in the S=1/2 Heisenberg antiferromagnet on
the square-kagome lattice, we study the S=1/2 Heisenberg antiferromagnets on
two other lattices composed of vertex-sharing triangles by the numerical
diagonalization method. One is a novel lattice including a shuriken shape with
four teeth; the other is the kagome lattice with
$\sqrt{3}\times\sqrt{3}$-structure distortion, which includes a shuriken shape
with six teeth. We find in the magnetization processes of these systems that a
magnetization jump accompanied by a spin-flop phenomenon occurs at the
higher-field-side edge of the magnetization plateau at one-third the height of
saturation. This finding indicates that the spin-flop phenomenon found in the
isotropic system on the square-kagome lattice is not an exceptional case.
