PURPOSE: To know the loading state of a tape reel to a reel hub before a magnetic tape device starts its working by detecting said loading state of the tape reel and informing it to a host device.

CONSTITUTION: A reel detecting part 2 contains a photocoupler consisting of a light emitting part 21 and a light receiving part 22. Both parts 21 and 22 are set at such positions where an optical axis L divides the gap H1 between covers 52 and 53 holding a tape reel 5 between them approximately into the equal parts and also divides the thickness H2 of a support ring 51 serving as the winding start part of a tape reel 5 approximately into two equal parts respectively. When the reel 5 is loaded to a reel hub, the axis L connecting both parts 21 and 22 is cut by the ring 51. Thus the output of the part 22 has a change. A reel detecting signal 201 is delivered from the part 2 and informed to a host device through an interface control circuit 1 in the form of a reel set signal 102.