An electric rotating machine including a rotor (50) having a spider arm (1) which is fixedly mounted on a substantially vertical shaft (11) and has arms (14) ex­tending radially outwardly from the shaft (11), and an annular rotor rim (2) fixedly mounted on distal ends of the arms (14) of the spider arm (1). The laminated core (3) is fixedly mounted on an outer periphery of the rotor rim (2), and is composed of a plurality of fan-shaped thin core plates (31) of steel laminated and arranged in an an­nular shape. Slot grooves (32) are formed in an outer pe­riphery of the core plates. The rotor rim (2) is composed of laminated rim members (21) each made of an annular thick steel plate, and rim engaging grooves are formed in an outer periphery of the rim member at equal intervals. The core plate has core engaging grooves (33) opposed to the rim engaging grooves (26), respectively, in such a manner that the number of complete core engaging grooves is two. An axially-extending connecting key (8) is fitted in each mating pair of core engaging groove and rim en­gaging groove to fix the laminated core (3) to the rotor rim (2).