An occupant judging device for a vehicle seat includes a load sensor provided in the vehicle seat and a controller for calculating a detected load value based on an output load value from the load sensor and determining an occupant on the vehicle seat by comparing the detected load value and a predetermined threshold value. The controller judges by a decrease state of the detected load value detected after the seatbelt is fastened being maintained for a first predetermined time that a child restraint device is installed, when a state in which the seatbelt is fastened and the door is opened is detected. Further, the controller judges by a decrease state of the detected load value detected after the seatbelt is fastened being maintained for a second predetermined time that the child restraint device is installed, when a state in which the seatbelt is fastened and the door is closed is detected.