A Monte Carlo simulation of solar neutron-decay protons is performed using a simple interplanetary magnetic field (slab model) in interplanetary space. The time profile of solar neutron-decay protons obtained by our simulation is compared with observations. It is shown that the simple model can predict coherent pulses at the start of events. Magnetic field parameters used in the comparison can suggest the mean free path of protons near the Earth. The values are discussed in relation to other published results. Also, the comparisons show that it is difficult to determine the angular distribution of solar neutrons emitted at a flare site as long as the presently available solar neutron-decay protons are used.