To provide a carrier device for a baby having multiple usage patterns and operation procedure therefor.

The carrier device for the baby includes a seat assembly, a backrest assembly including a first and a second backrest frames, and the backrest retention mechanism. The first backrest frame includes a first side part and a first crossing part, the second backrest frame includes the second side part and the second crossing part. The first side part is combined with the seat assembly via a first rotating point. The backrest retention mechanism is incorporated between the first crossing part and the second crossing part which can move, and includes a latch element which can move between the first position and the second position. The first position of the latch element makes the backrest retention mechanism to be in locking engagement with the first crossing part, and prevents the rotation of the backrest assembly to the seat assembly. The second position of the latch element releases the locking engagement of the backrest retention mechanism, and enables the rotation of the backrest assembly to the seat assembly.