Recently, magnetic devices utilizing a partial switching have been used in many fields. From a point of core design, a flux distribution must be known. It is possible to calculate them under some assumptions, but it is difficult to measure them experimentally. This paper describes a technique of determining a flux distribution in toroidal and multiaperture ferrite cores. The patterns of flux distribution are expressed by a flux of local zone. The flux level is determined by measuring a second-harmonic voltages induced in a pick up coil under an application of a small ac magnetic field (10 kHz) orthogonal to the plane of core. Small holes for sensing are drilled through the core, and the pick up coil encloses cross-sectional areas between each of the holes. It was found that a flux reversal boundary in a toroidal core travels from inner edge to outer edge when it is switched partially by a dc field, but a pulse field switching produces a complex flux distribution, and that in both cases, the flux reversal boundary is not sharp. By a similar method, the flux distribution in the transfluxor is shown for the "unblocked" and "blocked" state before and after the interrogation. The transfluxor in the unblocked state after the interrogration showed a "kidney shaped" pattern. Readout characteristics obtained from the measured flux distributions agree with those obtained by ordinary induced voltage methods. The accuracy of the readout flux is about 8.7 percent.