To improve workability in a protective device for a cylinder lock for a car with a casing with an inserting hole corresponding to a key hole for the cylinder lock, a shutter plate housed in the casing, enabling operation between the place of closing where the inserting hole is closed and the place of opening where the inserting hole is opened and a magnet lock having a fitting recessed section, in which a magnet key can be fitted in a manner that the magnet key can be inserted and detached, while being disposed to the casing so that the operation of the shutter plate from the closing place to the open place is permitted in the case of fitting the magnet key into the fitting recessed section.

An urging spring 48 in the closing direction urging the shutter plate 24A on the closing-place side is installed between the shutter plate 24A and the casing 18A, and an opening-place holding means 52 capable of holding the shutter plate 24A at an open place against the resilient force of the urging spring 48 is secured to the casing 18A.