Using the Ernest's method and adopting the Harrison transform under the linear condition, we obtained an expression of the exterior metric associated with the Kasuya dyon, q≠-2B 0J, in a magnetic field. There is an arbitrary parameter B0 in the expression, it expresses a homogeneously weak magnetic field. While r→∞, the metric approaches asymptotically the Melvin magnetic universe, it is reasonable that the metric does not become asymptotically flat in a homogeneously magnetic field.