The reaction of Mu with NO2 in a Kr atmosphere was studied by muon spin resonance and relaxation measurements at 300 K. It was found that the reaction proceeds via the following two steps : Mu+NO2-->MuO+NO and MuO+NO2-->MuNO(3). In particular, the final product MuNO(3) and the intermediate species MuO were successfully detected for the first time by the techniques of the muon spin resonance and the longitudinal relaxation under a high magnetic field of similar to 2800 G. The rate constants of the Mu+NO2 and the MuO+NO2 reactions were determined to be (2.9+/-0.5)X10(11) e mol(-1) s(-1) and (4.5+/-0.1)X10(9) e mol(-1) s(-1) at 300 K. The isotope effect in the rate constant of the Mu+NO2 reaction in comparison with the H+NO2 reaction suggests that there are reaction paths with a slight energy barrier which is high enough to retard the H reaction, but not the Mu reaction. On the other hand, the rate constant of the MuO+NO2 reaction is smaller than 50% of those of the OH+NO2 and the OD+NO2 reactions. This isotope effect seems to originate from faster rotational motion of MuO and/or lower vibrational density of states at the transition state.