To fit a pouring opening member correctly to the set fitting position for various packaging container main bodies.

A device has a conveyance means for conveying the packaging container main body PM, a fitting head for fitting the pouring opening member to the set fitting position of the main body PM, a head holding means for holding the fitting head, and a position adjusting means for adjusting the position of the fitting head in relation to the head holding means. The fitting head is held by the head bolding means, and the position of the fitting head is adjusted in relation to the head holding means by the position adjusting means. Even when the set fitting position of the container main body PM is not fixed, by adjusting the position of the fitting head, the set fitting position can be made to coincide always with an actual setting position so that the pouring opening member can be fitted correctly to the set fitting position for various packaging container main bodies.