PURPOSE: To obtain a cleaning soln. capable of chemically and effectively removing a degenerated layer on the surface of superinvar subjected to electric spark machining by adding hydrochloric acid and sulfuric acid each having a specified concn. to an aq. phosphoric acid soln. having a specified concn. and by heating them.

CONSTITUTION: Hydrochloric acid having 0.14W0.68mol concn. and sulfuric acid having 0.31W0.41mol concn. are added to an aq. phosphoric acid soln. having 1.53W1.63mol concn. and they are heated to 90W95°C to obtain a cleaning soln. for the surface of superinvar subjected to electric spark machining. When superinvar is immersed in the cleaning soln. after electric spark machining, a dark- or light-gray degenerated layer on the surface subjected to electric spark machining is easily and rapidly removed by ≤about 0.5μm without changing the surface roughness.