First-order structural phase transition based on charge order for Nd0.5Sr0.5MnO3 was studied by a newly developed low temperature and magnetic field X-ray diffraction apparatus. The structural phase transition that corresponds to a jump of lattice constants was confirmed even in magnetic fields up to 5 T. We observed the coexistence region consisting of high and low-temperature phases in magnetic fields. This region increased with increasing field. The domain of the high-temperature ferromagnetic phase remains in the charge order phase at low temperature and low magnetic field. The magnetic behavior is explained by the volume fraction of high-temperature domain.