  Quantum phase transitions between the magnetically ordered and disordered
states are studied for the two-dimensional antiferromagnetic quantum spin
systems with ladder, plaquette, and mixed-spin structures. Starting with
properly chosen singlet-cluster configurations, we perform the series expansion
for the staggered magnetic susceptibility. The phase boundary is determined by
applying the Dlog and biased Pad\'e approximants to the staggered
susceptibility thus obtained. The resulting phase diagram allows us to discuss
the quantum phase transitions quantitatively, which agrees fairly well with the
quantum Monte Carlo results for several cases previously studied.
