A precise measurement of the vector and axial-vector form factors difference $F_V-F_A$ in the decay $K^+→μ^+ν_μγ$ is presented. About 100K events of $K^+→μ^+ν_μγ$ have been selected in OKA experiment. The result is $F_V-F_A = 0.13 ± 0.02(stat.) ± 0.02(syst.)$. Both errors are two times lower of the first measurement of $F_V-F_A$ in ISTRA experiment. The presented result is considered as preliminary.