The position B-vg of a field-tuned superconductor-insulator quantum transition occuring in disordered thin films is examined within the mean field approximation and starting from a hamiltonian of BCS type (favoring the s-wave pairing) with a random potential, and effects of an electron-electron repulsion on the transition field B-vg are also considered. Our calculation shows that the (microscopic) disorder-induced reduction of B-vg suggested experimentally cannot be explained without taking account of the familiar interplay between the randomness and the electron-electron repulsion enhancing the quantum superconducting fluctuation in such systems.