The expressions describing the time-of-flight (TOF) and conditions of TOF focusing of ion packets by energies in the ion source with two accelerating gaps have been obtained. The first gap is the ionization region with a uniform electrostatic field, whereas the second gap is the immersion objective with a non-uniform electrostatic field, where the role of the “cathode” (emitting surface) is played by the exit window of the ionization region.Numerical calculations were used to obtain the ratios between geometric and electrical parameters of three-electrode immersion objectives with two-dimensional and rotational symmetry, determining conditions of four orders of TOF focusing by energy simultaneously with spatial focusing of ion packets. The schemes of time-of-flight mass spectrometers of high resolution and high sensitivity with direct and orthogonal ion injection are presented.