PURPOSE: To retrive the number of loading devices in a cassette-like magnetic bubble memory device, by utilizing a bubble select signal and a cassette insertion conforming signal.

CONSTITUTION: An inverting circuit 22 is connected to a bubble select signal line 20, inverting circuit 24, 25 whose mutual input terminals have been connected are connected to a cassette insertion confirming signal line 23, and to its input terminal, an output terminal of the inverting circuit 22 is connected. Also, to the signal line 23, an electric power supply V is connected through a resistance 27, and also a jumper pin 28 leading to earth is provided. Furthermore, in case when the number of loading devices is one, the pin 28 is turned on, and in case of two devices, it is turned off. When the pin 28 is on, the cassette insertion confirming signal Mon becomes "0" irrespective of a level of the bubble select signal Ds, and when the pin 28 is off, the signal Mon of "1" or "0" corresponding to the signal Ds is outputted. In this way, the number of loading devices is retrieved.