I. Introduction
The technologies of superconducting device utilizing vortices, such as superconducting vortex flow transistor (SVFT), Josephson vortex flow transistor (JVFT) and single flux quantum (SFQ) logic circuits, etc. have been remarkably progressed [1]–[3]. Especially SFQ logic circuits have the potential of ultra fast operation at the speed over hundreds GHz with very low power consumption compared to semiconductor logic circuits. Since these devices control vortices as carriers, it is very effective for analyzing and improving its performance, if we can directly observe the vortex states in the devices.