PURPOSE: To obtain the titled plating soln. giving deposited copper with superior physical properties, by adding a proper amount of a thallium compound to a plating soln. consisting of water, Cu ions, a complexing agent for Cu ions, a reducing agent for Cu ions, and a pH adjusting agent.

CONSTITUTION: An electroless copper plating soln. is obtd. by adding 10W100mg/ lthallium compound such as thallous nitrate to a plating soln. consisting of water, Cu ions to be deposited, a complexing agent for Cu ions, a reducing agent for Cu ions, and a pH adjusting agent. The Cu ions are fed from copper sulfate or the like. The complexing agent includes sodium ethylenediaminetetraacetate, the reducing agent includes formaldehyde, and NaOH or the like is suitable for use as the pH adjusting agent. Copper deposited using said plating soln. has superior physical properties such as superior luster, tensile strength and elongation, and copper deposited in a through-hole in a printed wiring board is prevented from cracking.