A vehicular curtain airbag apparatus and an attachment structure configured not to interfere with a driver seat airbag and being able to appropriately provide occupant protection performance either in normal and small overlap collision modes, and the curtain airbag having a simple configuration. The apparatus includes a main chamber having a front end to be deployed and inflated at a position near a front pillar, and an additional chamber further toward a vehicle front side and to be deployed and inflated, and separated from the main chamber by a partition portion between the front pillar and an inflation area of a driver seat airbag. The additional chamber is folded back near the partition portion toward the vehicle interior and a rear of the vehicle to overlap the main chamber, and parts of a resultant overlapping portion corresponding to upper portions of the main and additional chambers or vicinities of the parts are coupled together to form a chamber coupling portion.