A connecting device between components of a piece of furniture, including - An extended casing (10, 110, 210) along a longitudinal axis between a first end capable of being received in a first housing obtained in a first component of said piece of furniture and a second end capable of being received in a second housing obtained in a second component of said piece of furniture, said casing comprising a first region radially expandable in order to engage said first component in correspondence of said first housing, - An extended tie rod along said longitudinal axis and received inside said casing with axial sliding power towards and from a locking position, said tie rod comprising a first section in enlarged section susceptible of engagement with said first region to determine a radial expansion of said casing when said tie rod is moved to said locking position, - An eccentric element housed in said casing (10, 110, 210) and arranged to engage a head of said tie rod so as to axially move said tie rod towards said locking position when said eccentric element rotates around its main axis, wherein said casing (10, 110, 210) comprises a second radially expandable region to engage said second component of the piece of furniture in correspondence of said second housing and said tie rod comprises a second segment in enlarged section susceptible of engagement with said second region to determine a radial expansion of said casing when said tie rod is moved to said locking position, wherein said casing (110, 210) comprises: - a first component (111, 211) associated with said radially expandable first region, - a second component (112, 212) associated with said radially expandable second region, said first component (111, 121) of the casing (110, 210) being axially translatable with respect to said second component (112, 212) of the casing (110, 210) between a mounting position, in which said first component (111, 121) of the casing (110, 210) is axially spaced from said second component (112, 212) of the casing (110, 210), and a clamping position, in which said first component (111, 121) of the casing (110, 210) is axially approached to said second component (112, 212) of the casing (110, 210), - a provisional restraining element (117, 217) arranged between said first component (111, 121) and said second component (112, 212) of the casing (110, 210) and configured to preserve the mutual axial position between the first (111, 211) and the second component (112, 212) of the casing (110, 210) during an initial expansion phase of the radially expandable first region, and wherein said first component (111, 211) of the casing (110, 210) is received having limited possibilities of axial sliding in a housing (115, 215) obtained in said second component (112, 212) of the casing (110, 210). - An extended casing (10, 110, 210) along a longitudinal axis between a first end capable of being received in a first housing obtained in a first component of said piece of furniture and a second end capable of being received in a second housing obtained in a second component of said piece of furniture, said casing comprising a first region radially expandable in order to engage said first component in correspondence of said first housing, - An extended tie rod along said longitudinal axis and received inside said casing with axial sliding power towards and from a locking position, said tie rod comprising a first section in enlarged section susceptible of engagement with said first region to determine a radial expansion of said casing when said tie rod is moved to said locking position, - An eccentric element housed in said casing (10, 110, 210) and arranged to engage a head of said tie rod so as to axially move said tie rod towards said locking position when said eccentric element rotates around its main axis, - a first component (111, 211) associated with said radially expandable first region, - a second component (112, 212) associated with said radially expandable second region, said first component (111, 121) of the casing (110, 210) being axially translatable with respect to said second component (112, 212) of the casing (110, 210) between a mounting position, in which said first component (111, 121) of the casing (110, 210) is axially spaced from said second component (112, 212) of the casing (110, 210), and a clamping position, in which said first component (111, 121) of the casing (110, 210) is axially approached to said second component (112, 212) of the casing (110, 210), - a provisional restraining element (117, 217) arranged between said first component (111, 121) and said second component (112, 212) of the casing (110, 210) and configured to preserve the mutual axial position between the first (111, 211) and the second component (112, 212) of the casing (110, 210) during an initial expansion phase of the radially expandable first region,