A holder-mounting structure comprises a holder and a holder mounting portion into which the holder is slidably inserted. The holder includes a holder body for mounting of an electric component and a pair of plate portions extending from the holder body such that the electric component resides therebetween. The plate portions include a thick-walled portion continuing to the holder body and a thin-walled portion continuing to the thick-walled portion and provided at an end of the plate portion away from the holder body. The holder mounting portion includes an accommodating portion into which a connecting terminal connected to the electric component is accommodated. The thin-walled portion includes a complete-locking portion configured to be brought into locking engagement with the holder at a position where the electric component is connected to the connecting terminal with the holder slidably inserted into the accommodating portion.