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Magnetic excitations in an intermetallic compound MnSi have been studied by neutron scattering. At 5 K in a magnetic field of 10 kOe, well-defined spin-wave excitations have been observed 0 belo~2.5 meV. The dispersion relation is almost isotropic and is hm"(meV) =0.13+52@2 (A 2) in the [100] direction, Above 3 meV, the excitation linewidth increases substantially, suggesting that the dispersion relation merges into the Stoner continuum. The Stoner excitations, which extend over almost all of the Brillouin zone, show a broad peak on the extension of the spin-wave dispersion relation. The spin-wave excitation renormalizes with increasing temperature and collapses into critical scattering above 40 K. On the other hand, the excitation in the Stoner continuum is aft'ected little by temperature; the excitations are qualitatively the same at T/T~=10 as at 5 K. The Stoner boundary energy decreases with increasing temperature. The results provide us with the first example of magnetic excitations in a weak itinerant ferromagnet.