Security device adapted to be coupled to a cylinder lock, said security device comprising a casing associable to said lock and provided with a first axial opening adapted to house a cylinder of said lock, said first axial opening being eccentrically arranged in respect to the center of said casing, a protective key-plate adapted to be coupled to said casing, said protective key-plate being provided with a second through-opening eccentrically arranged in respect to the center of said protective key-plate such that said second through-opening is aligned, in use, with said first opening, a slider movable in respect to said casing between a locking position, wherein said slider occludes said first opening in order to prevent, in use, accessing said cylinder, and an unlocking position, wherein said slider allows accessing said first opening, and a magnetic combinatory unit (17, 18, 19, 20, 21, 22, 23, 24) movable together with said slider, a cavity facing, in use, said casing being provided on an inner surface of said protective key-plate, said cavity having a substantially linear extension and being arranged substantially parallel to the medial axis passing through said second through-opening and said protective key-plate, guide means (10, 11) being provided on each side of said cavity, said guide means (10, 11) comprising a first guiding surface arranged at a side of said cavity and a second guiding surface arranged at the opposite side of said cavity, said guide means (10, 11) including engaging means (12, 13, 14, 15) comprising at least a first recess and a second recess provided on said first guiding surface, and at least a third recess and a fourth recess provided on said second guiding surface, said slider comprising blocking means (25, 26) adapted to engage said engaging means (12, 13, 14, 15) to firmly block said slider into said locking position or unlocking position, said blocking means (25, 26) including a pair of pins (27, 28) provided with respective heads (30A, 30B) having a shape complementary to the shape of said engaging means (12, 13, 14, 15), said heads (30A, 30B) being adapted to selectively engage said first recess and said third recess, when said slider is in said locking position, and said second recess and said fourth recess when said slider is in said unlocking position, characterized in that said magnetic combinatory unit (17, 18, 19, 20, 21, 22, 23, 24) includes at least four satellites (17, 18, 19, 20), namely a first satellite, a second satellite, a third satellite and a fourth satellite each containing respective magnetic means (21, 22, 23, 24), said at least four satellites (17, 18, 19, 20) being adapted to be rotatably housed into respective seats (34, 35, 36, 37) provided on said slider, said seats being mutually arranged at right angles, wherein said first recess axially faces said third recess and said second recess axially faces said fourth recess, said blocking means (25, 26) being controlled by said at least four satellites (17, 18, 29, 20), wherein each of said first satellite, second satellite, third satellite and fourth satellite comprises one respective cut (38, 39, 40, 41), said blocking means (25, 26) further comprising a first flared portion and a second flared portion respectively connecting said heads (30A, 30B) to the respective pins (27, 28), said first flared portion being adapted to selectively cooperate with the respective cuts (38, 40) of said first satellite and said third satellite and said second flared portion being adapted to selectively cooperate with the respective cuts (39, 41) of said second satellite and said fourth satellite.