A connector housing has terminal accommodating cavities therein and a first engagement projection in each of the cavities. A terminal locking block has a locking claw on a preliminary locking plate that loosely connects the block to the connector housing. The block also has second engagement projections on a flexible terminal locking plate. When the terminals are inserted into the cavities of the connector housing, the front end of the terminals abuts against the end of the flexible terminal locking plate and thereby pushes the block partly out of the housing. When the terminals are fully inserted, a space is formed in the cavities so that the block can be pushed into the housing until the second engagement projections snap into the engagement holes in the inserted terminals, thus locking the terminals and the block securely together. Since the block can be fully locked only when the terminals are fully inserted in the correct positions and because during the terminal insertion process the block is protruded partly out of the housing, any incomplete insertion of terminals can be checked and prevented.