For ICRH scenarii having the majority cyclotron resonance intersecting the plasma core, mode conversion of the fast magnetosonic wave into an Alfvén wave at the plasma boundary on the high field side takes place. Simple analytical estimates for the convened power in this mode conversion process are derived. In addition to the Budden parameter the convened power is found to depend only on the local absolute value of the fast wave electric field at the Alfvén resonance. The effect of the reflection of the fast wave from the wall surrounding the plasma on the conversion is then easily explained. The results are compared with numerical calculations where finite electron inertia and kinetic effects are included. The mode conversion is strongest for weak local density gradient, high density, and for resonances lying not too near the wall. Its dependence on parallel wavenumber turns out to be sensitive to the ion composition and on the exact location of the resonance. For first harmonic heating of tritium in a deuterium-tritium plasma in JET tokamak a nearly complete absorption of the power reaching the conversion layer is predicted for low parallel wavenumbers. This is a serious problem because of the weak absorption in the centre for this scenario.