Abstract: In a connection structure of a rigid printed circuit board and a flexible circuit each having a plurality of connection terminals, there are provided a connection structure that can obtain a necessary connection strength and prevent short-circuiting between adjacent connection terminals, and a connection process thereof. A rigid printed circuit board having a plurality of connection terminals is superimposed on a flexible circuit that puts a conductive pattern having connection terminals having the same configuration as that of the connection terminals of the rigid printed circuit board at an end thereof by flexible insulating resin, and connected to the flexible circuit with a solder due to thermo compression. A solder resist is disposed on the flexible insulating resin on the flexible circuit, and the solder connection can be realized by using a solder plating formed on one or both electrodes of the rigid printed circuit board and the flexible circuit while an amount of occlusion gas is controlled.