A multi-configuration self-consistent field method is proposed in which a quasi-complete active space, which is the product space of complete active spaces, is used as a variational space. The computational effort for wavefunctions is significantly reduced compared with the complete active space self-consistent field (CAS-SCF) method. Efficient formulae with segmented alpha and beta strings are derived. The scheme is tested on the spectroscopic constants and a potential energy curve of CO, the transition state barrier height of the reaction, H2CO --> H-2 + CO, and the energy difference between trans- and cis-butadiene. The results are in good agreement with the corresponding CAS-SCF results.