Resonant tunneling process in an (InGa)As-(InAI)As symmetric double-barrier structure subjected to a transverse magnetic field B⊥ (perpendicular to the current) is investigated. We particularly focus on the experimental behaviour of Vp, the voltage at the current peak position, as a function of the magnetic field B⊥. For strong magnetic fields a clear dependence on B⊥2 is observed, as expected. However, an original result is obtained at small magnetic fields where a deviation from the parabolic behaviour is observed. The more complicated dependence on B⊥ of Vp is obtained from a numerical fit. A qualitative discussion is proposed on the basis of a previous publication (L. A. Cury, A. Celeste, J. C. Portal, Solid-States Electron. 32 (1989) 1689) and the differences with the results of other authors are pointed out.