PURPOSE: To improve a charging function by preventing a charging terminal from the adhesion of dust or the like, suppressing deterioration in the contact property of the charging terminal to heighten contact reliability.

CONSTITUTION: Charging terminal exposing windows 12 are formed on positions respectively corresponding to master equipment side charging terminals 8 on the bottom 10 of a slave equipment 2 and slave equipment side charging terminals 14 are formed on the insides of the windows 12. A closing member 16 having a function for normally closing the windows 12 is arranged on the inside of the windows 12. The member 16 is constituted of a protection cover 28 applied so as to be optionally rotated and a twisted coil spring 30 for exciting the cover 28 in the closing direction.