An object is to provide a surface treating agent, which in mounting electronic parts or the like to a printed wiring board using a lead-free solder, forms a chemical layer having excellent heat resistance on the surface of copper or a copper alloy constituting a circuit part of a printed wiring board or the like and at the same time, improves the wettability to the solder and makes the solderability good, and a surface treatment method. Also, another object is to provide a printed wiring board resulting from bringing the surface of copper or a copper alloy constituting a copper circuit part into contact with the foregoing surface treating agent and to provide a soldering method by bringing the surface of copper or a copper alloy into contact with the foregoing surface treating agent and then performing soldering using a lead-free solder. A surface treating agent for copper or a copper alloy, which contains an imidazole compound represented by the formula (I): wherein R1, R2 and R3 are the same or different and represent a hydrogen atom or an alkyl group having 1 to 8 carbon atoms; and at least one selected from R1, R2 and R3 is an alkyl group having 4 or more carbon atoms.