The distributions of optical emission intensity along the discharge axis in multiple‐parallel‐plate ac glow discharges were measured in the presence of a magnetic field perpendicular to the discharge electric field. In a SiH4(10%)/Ar discharge, time‐averaged spectroscopic measurements showed that profile control of not only plasmas but also SiH* was possible by varying the field strength. The emission intensity of SiH* was increased with the field strength. These results suggest that the present new configuration with cross magnetic field can make thin films on a large‐area substrate outside the multielectrodes uniform and can enhance the deposition rate.