  A hydrogen-like atom consisting of a positive muon and an electron is known
as muonium. It is a near-ideal two-body system for a precision test of
bound-state theory and fundamental symmetries. The MuSEUM collaboration
performed a new precision measurement of the muonium ground-state hyperfine
structure at J-PARC using a high-intensity pulsed muon beam and a high-rate
capable positron counter. The resonance of hyperfine transition was
successfully observed at a near-zero magnetic field, and the muonium hyperfine
structure interval of ${\nu}_{\text{HFS}}$ = 4.463302(4) GHz was obtained with
a relative precision of 0.9 ppm. The result was consistent with the previous
ones obtained at Los Alamos National Laboratory and the current theoretical
calculation. We present a demonstration of the microwave spectroscopy of
muonium for future experiments to achieve the highest precision.
