A conversion plug has a pair of guide projections that hold therebetween a pair of blades in a stand state at the time of attachment of the conversion plug to an attachment plug, and guide the pair of blades into a pair of recesses. A plug main body has, at positions respectively adjacent to the pair of blades in the stand state, a pair of storage recesses for housing the pair of guide projections. During assembly, the pair of blades are naturally guided to the pair of recesses by the guide projections without an operator's specific care. After the assembly, the pair of blades are engaged in the pair of recesses and the guide projections are engaged in the storage recesses so that the guide projections cover the pair of blades thereby preventing the pair of blades from being deformed.