To provide a hand strap mounting structure with which a user can easily attach/detach various characteristic hand straps without distinguishing them from one another and can strongly hold the hand straps.

In a mobile terminal device 1 wherein a casing for a main body device is composed of a front casing 2, a rear casing 3 and a battery cover 4, a hand strap mounting part 6 is provided in a lower portion of the rear casing 3. The hand strap mounting part 6 is configured by forming a U-shaped groove 6-1, in which a ring-like thin string 5-1 of the hand strap 5 can be housed, at a position in contact with an outer peripheral area of the rear casing 3. In such a case, at a position where an opening of the U-shaped groove 6-1 is brought into contact with a side wall part of the rear casing 3, two holes are formed in accordance with the groove depth of the U-shaped groove 6-1. Furthermore, in the battery cover 4, a thin string pressing rib 4-4 is provided at a position where the thin string is fitted into the U-shaped groove 6-1 provided on the rear casing 3 in a state where the battery cover 4 is mounted.