A device (2, 2') for automatic closing of a door, to be applied to a hidden hinge comprising two bodies (8, 10) which are articulated with each other by means of at least one pair of arms (48, 60) and are constrainable to a leaf and to a fixed structure, respectively, said device (2, 2') being intended to be constrained to one of said two bodies of said hinge and to be inserted, together with said body, into a corresponding cavity obtained in said leaf, said device (2, 2') comprising: - a translating element which can be engaged with an articulation arm of said hinge, - a first element and a second element, - elastic means interposed between said first element associated with said translating element and said second element associated in use with said body of hinge to which said device (2, 2') is intended to be constrained, said elastic means being configured to be loaded during the opening rotation of said leaf associated with said hinge and to return the mechanical energy stored in said opening step to said translating element in order to cause the automatic closing of said leaf associated with said hinge, - means (50, 51, 52, 53, 55, 57) for varying the preloading of said elastic means, and therefore the automatic closing speed of said leaf, said means (50, 51, 52, 53, 55, 57) being configured to modify the distance between said first and said second elements between which said elastic means are interposed, wherein said means (50, 51, 52, 53, 55, 57) for varying the preloading of said elastic means can be controlled from a base of said body of hinge which is visible also once said closing device (2, 2') has been inserted into said cavity obtained in said leaf, characterized in that said means (50, 51, 52, 53, 55, 57) for varying the preloading of said elastic means comprise means (50, 51, 52, 53, 55, 57) for modifying, when adjusting said device (2, 2') for closing a door, the distance between said second element and the lower base of said body of hinge to which said device (2, 2') is intended to be constrained. - a translating element which can be engaged with an articulation arm of said hinge, - a first element and a second element, - elastic means interposed between said first element associated with said translating element and said second element associated in use with said body of hinge to which said device (2, 2') is intended to be constrained, said elastic means being configured to be loaded during the opening rotation of said leaf associated with said hinge and to return the mechanical energy stored in said opening step to said translating element in order to cause the automatic closing of said leaf associated with said hinge, - means (50, 51, 52, 53, 55, 57) for varying the preloading of said elastic means, and therefore the automatic closing speed of said leaf, said means (50, 51, 52, 53, 55, 57) being configured to modify the distance between said first and said second elements between which said elastic means are interposed, wherein said means (50, 51, 52, 53, 55, 57) for varying the preloading of said elastic means can be controlled from a base of said body of hinge which is visible also once said closing device (2, 2') has been inserted into said cavity obtained in said leaf, characterized in that said means (50, 51, 52, 53, 55, 57) for varying the preloading of said elastic means comprise means (50, 51, 52, 53, 55, 57) for modifying, when adjusting said device (2, 2') for closing a door, the distance between said second element and the lower base of said body of hinge to which said device (2, 2') is intended to be constrained.