The paper presents a method of multi-parameter numeric optimization of plane constructions of a constant thickness made of a linearly elastic homogeneous material. The optimization algorithm is based on stigmergy which is becoming increasingly popular in combinatorial optimization and lately also in multi-parameter optimization. The optimization criteria is a function built on the basis of a frequency spectrum of an arbitrary range determining stiffness of the plane structure. The method was verified on a real industrial optimization problem of an electro-motor casing.