We have fabricated a superconducting vortex flow transistor (SVFT) device with a YBa(2)Cu(3)O(7-delta) (YBCO) thin film on an MgO substrate. To detect the local magnetic flux state inside the superconducting loop of the device, we utilized a scanning laser magneto-optical microscope (SLMO). As a result we have succeeded in the observation of magnetic flux distribution in the device by applying dc-control current larger than 100 mA. Moreover, we could detect the time variation of local magnetic field in the superconducting loop as the MO signals by applying ac-control current with an amplitude of 100 mA at 500 Hz.