MnP exhibits superconductivity under pressure with a maximum transition temperature of ~1 K at 8 GPa. Since Mn has the spin degree of freedom, elucidating the magnetic contribution to the superconductivity is crucial to understand the pairing mechanism. We performed neutron diffraction experiments in MnP under high pressure and determined the magnetic phase diagram as a function of pressure. We found that a helical magnetic structure is in the vicinity of the superconducting phase, which is similar to the magnetic structure in the sister material CrAs. In this study, challenging high-pressure neutron diffraction measurements were performed. The high-pressure techniques, including the recent development, are also shown in this article.