Abstract: The invention provides a nickel (alloy) electroplating solution which can fill minute holes or minute recesses 14 in the electronic circuit components 14 with the nickel or nickel alloy 18 without producing voids or seams, and can join two or more electronic components firmly together by filling minute gaps. The invention further provides a method of nickel or nickel alloy plating and filling using the aforesaid nickel (alloy) electroplating solution, a method of manufacturing a minute 3-dimensional structure, an electronic component assembly, and a method of manufacturing the same. The aforesaid problems in filling the minute holes or minute recesses 14 are overcome by using a nickel (alloy) electroplating solution containing a specific N-substituted pyridinium compound.