The two‐dimensional transonic flow of a perfectly conducting, inviscid, compressible fluid past a thin body with aligned magnetic field is studied by developing a small‐perturbation theory in the hodograph plane. It is shown that the equations of motion as well as the conditions for possible shock waves can be reduced to those of ordinary flow by a suitable affine‐transformation. Thus, von Ka´rma´n's transonic similarity law is extended to the present class of magneto‐gas‐dynamic flow. In this extension, super‐Alfve´nic flows (flow speed larger than Alfve´n wave speed) are found to be similar to the corresponding ordinary flow, while sub‐Alfve´nic flows are related to the ordinary flow with reversed flow direction. The flow over a half‐wedge at Mach number 1 is considered in detail.