The influence of magnetic field on confined exciton-polariton modes inside a semiconductor microcavity is discussed. The three-dimensional confinement for exciton-polaritons is achieved by the mesa structures confining the photonic part of polaritons. We observe a strong increase of the polariton emission intensity and we argue that this effect is due to the change of the oscillator strength of the excitonic component of polaritons and the change of the excitonic content in polariton state as the magnetic field increases.