  The mixed diamond chain is a frustrated Heisenberg chain composed of
successive diamond-shaped units with two kinds of spins of magnitudes S and S/2
(S: integer). Ratio $lambda$ of two exchange parameters controls the strength
of frustration. With varying $lambda$, the Haldane state and several spin
cluster states appear as the ground state. A spin cluster state is a tensor
product of exact local eigenstates of cluster spins. We prove that a spin
cluster state is the ground state in a finite interval of $lambda$. For S=1, we
numerically determine the total phase diagram consisting of five phases.
