Child car seat

A child car seat is capable of being easily set on a passenger seat (7) of a vehicle and of simply and sufficiently tightening a seat belt (4). The child car seat to be fixedly held on a passenger seat (7) of a vehicle by a seat belt (4) has a seat belt fastening mechanism (20) for fastening a diagonal section (4a) of the seat belt (4), a seat belt tightening mechanism (11) for deflecting a lap section (4b) of a fixed length of the seat belt (4), and a locking mechanism (15) for locking the seat belt tightening mechanism (11). The lap section (4b) of the seat belt (4) extended across a base (10) is deflected and is kept in a deflected state to tighten the seat belt (4).