PURPOSE: To prevent the coarsening of the recrystallized structure in a beryllium copper alloy in a heating stage in soln. treatment by executing preheating under specified conditions before subjecting a plastic working material of a beryllium copper alloy having a specified compsn. to soln. treatment.

CONSTITUTION: A plastic working material of a beryllium copper alloy consisting essentially of, by weight, 1.6 to 2.0% Be and 0.2 to 0.6% (Co+N) and the balance Cu is subjected to preheating in the temp. range of 500 to 650°C for ≥1hr (about ≤20hr) before heating it to ≥700°C and executing soln. treatment or again executing hot working. In this way, at the time of heating the material to ≥700°C for soln. treatment or the like, the coarsening of its recrystallized structure can be prevented and its structure can be formed into a uniform and fine crystalline one having about ≤2mm average grain size, by which a beryllium copper alloy material satisfying the reliability in mechanical characteristics and ultrasonic flaw detecting properties can be obtd.