Abstract: A head with a pick-up coil is attached to a supporter so that the head is freely movable along the direction perpendicular to the surface of a superconductor. Fluid spouted from the spout hole of the head maintains the height (gap space) of the head from the surface. The head is firmly fixed to the supporter in the direction along the surface. Since the relative position of the head in the direction along the surface of the superconductor is changable by drive means for moving the superconductor or the head, the distribution of the magnetic flux which is trapped in the superconductor is measured by scanning the surface of the superconductor with pick-up coil.