The muonic helium-4 atom is an atom in which one of the two electrons of the 4He atom is replaced by a negative muon. Spectroscopy of muonic helium-4 atom at high magnetic field will allow us to precisely determine negative muon magnetic moment and mass. To verify our spectroscopy apparatus and method, we performed a zero magnetic field spectroscopy and got a result with a world-leading precision of 4.5 ppm. The main limitation is statistical uncertainty, influenced by beam intensity and by muon spin depolarization during muonic helium-4 atom formation. Toward higher precision, we are planning to incorporate the SEOP technique which allows us to repolarize negative muon. I will introduce results of spectroscopy of muonic helium and negative muon reporalization test at J-PARC.