To prevent the burglar of a public telephone set due to the break-open of a cashbox, the cutting of a padlock attached to a cashbox lock device, etc., of a public telephone set, by preparing a protective structure where the cashbox and the padlock attached to the cashbox lock device are covered.

This device consists of a base 21 which is placed between a public telephone set and a cabinet slide plate, a cover 22 which is removably attached to the base 21 and covers a cashbox and a padlock attached to a cashbox lock device of the telephone set, and a lock 23 which connects and fixes the base 21 and the cover 22 together. The cover 22 uses an L-shaped steel plate and includes a planar lock attachment part which extends horizontally toward the inner surface of a plane part B and has a hole where a rod 24 of the lock 23 is inserted. The rod 24 can be loaded into the lock 23 via the operation of a key 32. Then the cover 22 and the base 21 are fit together and then connected and fixed together by the lock 23. Thus, no gap is substantially produced between such burglar prevention device and the telephone set when the former is attached to the latter.