Applying a solenoidal magnetic field to a laser ion source is a method to produce a high current beam. In this study, we measured the effect of the solenoidal magnetic field on the time evolution of the ion beam emittance using the double-slit method for the laser ion source. Ablation plasma was produced by irradiating an Al target with an Nd:YAG laser. From the emittance measurements, the phase difference shown in the time evolution of the emittance ellipses increased as the ion current increased by applying a magnetic field. In addition, the emittance increased with increasing time range for the averaged current in the waveform. However, the emittance using the averaged current in a pulse was almost constant with the magnetic field. These results indicate that the brightness of the beam increased as the ion beam current increased using the solenoidal magnetic field.