PURPOSE: To realize a detector area which obtains a large detecting efficiency stably by arranging an auxiliary pattern composed of a soft magnetic material thin film along the magnetic bubble detector into a magnifying device.

CONSTITUTION: The auxiliary pattern 8 in parallel to a straight line detector 1 is arranged in a magnifying device 3. The pattern 8 is formed simultaneously with the detector 1 composed of the soft magnetic material thin film and a cusp part 5 is located almost on the central line of the pattern 8. A magnetic bubble is transferred in the C direction along a transfer line 2 and arrives at the cusp part 5. At this time, an expanded pulse electric current is loaded by the phase of an in-surface rotating magnetic field and then an absorbing magnetic field occurs at the B side of the detector and the magnetic bubble is extended with the B side end of the detector 1 as a center.