The connecting rods employed in most of general-purpose engines with a power from 1.5 kW to 10 kW are manufactured from aluminum alloy in order to increase productivity and reduce weight, and therefore display lower material strength than steel connecting rods. In terms of operating conditions, general-purpose engines are frequently operated under high load while being held at a comparatively low engine speed, necessitating strength and durability in relation to combustion pressure. Realizing a balance between the rigidity of each part is an important factor in reconciling the achievement of weight reduction in the reciprocating parts while also keeping strength. The research discussed in this paper developed a structure optimization system to examine shape parameters for reciprocating parts targeting 4 kW class engines, and studied shapes that would balance the reduction of weight with the keeping of strength. These realizes design shapes of 4 kW class engines in which the stress generated on each part is equivalent to or lower than the stress generated on the mass-production shape, while reducing the mass of reciprocating parts by approximately 6% (compared to the initial shape). Investigation using reciprocating shapes of general-purpose engines with different power showed that shapes of reciprocating parts with the smallest weight had the same dimension pattern as discussed above irrespective of the cylinder bore size.