Neutrinos play a significant and sometimes even dominant role in all phases of the supernova explosion. The most important neutrino processes in a core-collapse supernova are beta-processes, which are responsible for the energy exchange between neutrinos and the matter and change a chemical composition of a medium. We investigate an influence of a magnetic field on beta-processes under conditions of a supernova matter. For any magnetic field strength discussed in applications to astrophysical objects we obtain simple analytical expressions for reaction rates of beta-processes. In our analysis we use results of one-dimensional simulations of a supernova explosion performed with the PROMETHEUS-VERTEX code. We found that, the magnetic field with the strength $B \sim 10^{15}$ G suppresses the neutron production on several percents in comparison with the unmagnetized case. The obtained analytical expressions can be also applied for postmerger accretion discs.