Ising model on decorated square lattice is studied with arbitrary signs and values of exchange parameters in an external magnetic field. Comparison with the results of Ising madel on nondecorated square lattice is performed. It is shown that if magnetization increases antiferromagnetically then initial phase-transition-point gradually decreases up to the first frustration field and does not appear again even if further frustration fields apear. In the case of ferromagnetic type of magnetization increasing the phase transition disappears right away after switching on of magnetic field. In other words, an arbitrary low field completely suppresses the phase transition.