The K^+ \to \pi^0 \mu^+ \nu \gamma (Kμ3γ​) decay has been
measured with stopped positive kaons at the KEK 12 GeV proton synchrotron. A
Kμ3γ​ sample containing 125 events was obtained. The partial
branching ratio Br(Kμ3γ​,Eγ​>30MeV,θμ+γ​>20∘) was found to be [2.4±0.5(stat)±0.6(syst)]×10−5, which is in good agreement with theoretical predictions.Comment: 12 pages, 3 figures, to be published in Physics Letters