The microscopic magnetism in the helical, conical and ferromagnetically polarized phases in an itinerant helical magnet, MnSi, has been studied by an extended 29Si NMR at zero field and under external magnetic fields. The temperature dependence of the staggered moment, MQ (T), determined by the 29Si NMR frequency, $\nu$(T) and the nuclear relaxation rate, 1=T1 (T), at zero field is in general accord with the SCR theory for weak itinerant ferromagnetic metals and its extension to helical magnets. The external field dependence of resonance frequency, $\nu$(H), follows a vector sum of the contributions from the atomic hyperfine and macroscopic fields with a field induced moment characteristic to itinerant magnets. A discontinuous jump of the resonance frequency at the critical field, Hc , between the conical and the polarized phases has also been found, which suggests a first order like change of the electronic states at Hc.