The axial dependence of the anode current distribution in a 2.45-GHz oven magnetron was investigated. We previously demonstrated that installing a cathode shield is an effective way to reduce magnetron noise. The objective of the present study is to determine why installing a cathode shield reduces the noise. We measured the anode current of a test magnetron with an axially segmented anode block. When no magnetic field was applied, the test magnetron behaved like a cylindrical vacuum diode, and the axial electron distribution corresponded to the axial distribution of the cathode temperature. When a magnetic field was applied, the cathode shield increased the normalized anode current in the central region, which contributes to noise reduction.