PURPOSE: To control the thickness of an electroless plating film at a relatively low cost and with sufficient precision by detecting the progress of plating from a change in the liq. surface with a partitioned section communicating with a residual section through a metallic mesh dipped in an electroless plating soln.

CONSTITUTION: A metallic mesh 3 laid over the bottom opening of a vessel 2 is dipped in an electroless plating soln. 1 to form a partitioned section A communicating with a residual section B only through the mesh 3. The plating soln. 1 in the residual section B is then introduced by a feed pump 4 into the vessel 2 from its upper opening, and the plating soln. 1 is circulated between the sections A and B through the mesh 3. As electroless plating proceeds, the liq. surface 21 of the section A is raised due to the clogging of the mesh 3. The clogging of the mesh 3 accurately corresponds to the thickness of a plating film on a body to be electroless-plated. Accordingly, the ascension of the liq. surface 21 is measured to detect the progress of plating, and the thickness of the plating film is controlled.