  We develop a mean-field theory for random quantum spin systems using the spin
coherent state path integral representation. After the model is reduced to the
mean field one-body Hamiltonian, the integral is analyzed with the aid of
several methods such as the semiclassical method and the gauge transformation.
As an application we consider the Sherrington-Kirkpatrick model in a transverse
field. Using the Landau expansion and its improved versions, we give a detailed
analysis of the imaginary-time dependence of the order parameters. Integrating
out the quantum part of the order parameters, we obtain the effective
renormalized free energy written in terms of the classically defined order
parameters. Our method allows us to obtain the spin glass-paramagnetic phase
transition point $\Gamma/J\sim 1.62$ at T=0.
