Recently generation of strong magnetic (B) fields has been demonstrated in capacitor coils heated by high power laser pulses [S. Fujioka et al., Sci. Rep. 3, 1170 (2013)]. This paper will present a direct measurement of B field generated with an open-ended coil target driven by a nanosecond laser pulse using ultrafast proton radiography. The radiographs are analyzed with particle-tracing simulations. The B field at the coil center is inferred to be ~50 T at an irradiance of -5×10¹⁴ W?cm². The B field generation is attributed to the background cold electron flow pointing to the laser focal spot, where a target potential is induced due to the escape of energetic electrons.