To provide a connection device where the spring properties of nickel plating layers particularly composing contacts can effectively be improved, and further, the composition of a plating bath and conditions are optimized for obtaining the properties, and to provide its production method.

A 1 to 4(g/L) of brightener such as saccharin is added to a watt bath used for plate-forming Ni plating layers 45. Further, the current density is controlled to the range of 1 to 3 (A/dm2), and the Ni plating layers 45 are plate-formed inside patterns 42a. In the Ni plating layers 45, the tensile strength reaches 900 to 1,600 (MPa), and the Young's modulus reaches 150 to 230 (GPa), by which the spring properties of spiral contacts 20 having the Ni plating layers 45 can be improved.