A seatbelt alarm device for a vehicle includes a seat sensor, a seatbelt sensor, a judgment circuit, an alarm unit, and a child seat sensor. The seat sensor detects a mass on a seat of the vehicle. The seatbelt sensor detects whether a seatbelt of the seat is engaged with a seatbelt buckle. The judgment circuit generates a demand for a seatbelt-wear alarm upon determining that a mass sits on the seat and the seatbelt is not engaged according to detection signals of the seat sensor and the seatbelt sensor, respectively. The alarm unit generates the seatbelt-wear alarm. The child seat sensor detects a child seat mounted on the seat. The judgment circuit restricts operation of the alarm unit when the judgment circuit determines that the child seat is mounted to the seat according to a detection signal of the child seat sensor.