An occupant decision device for a vehicle that has a vehicle seat provided with a seat belt device of which one end is fixed to a vehicle floor, the occupant decision device comprising: a plurality of load detecting bodies (4FR, 4RR, 4FL, 4RL) that respectively detect seat loads (Wfr, Wrr, Wfl, Wrl) of a left portion and a right portion of the seat; an occupant decision section that decides a type of occupant on the vehicle seat from at least one of a state where the vehicle seat is vacant, a state where a child seat is attached, or a state where an adult is seated based on the seat loads detected by the load detecting bodies, and maintains or changes a decision result of the type of the occupant; a seat belt wearing detection body that detects a tongue of the seat belt device being engaged with a buckle; a buckle load calculation section that calculates a buckle load (Wb), which is a load of a side where the buckle is disposed in the portion of the seat, based on the seat loads detected by the load detecting bodies; and a child decision section that prohibits the occupant decision section from changing a decision to a decision result that an adult is seated in the vehicle seat and changes the decision to a decision result that the child seat is attached on the vehicle seat when an engagement of the tongue with the buckle is detected by the seat belt wearing detection body and the buckle load calculated by the buckle load calculation section is less than a predetermined buckling load threshold (Wbth1, Wbth2, Wbth3) in a case where the occupant decision section decides that an adult is seated in the vehicle seat based on the seat loads detected by the load detecting bodies. a plurality of load detecting bodies (4FR, 4RR, 4FL, 4RL) that respectively detect seat loads (Wfr, Wrr, Wfl, Wrl) of a left portion and a right portion of the seat; an occupant decision section that decides a type of occupant on the vehicle seat from at least one of a state where the vehicle seat is vacant, a state where a child seat is attached, or a state where an adult is seated based on the seat loads detected by the load detecting bodies, and maintains or changes a decision result of the type of the occupant; a seat belt wearing detection body that detects a tongue of the seat belt device being engaged with a buckle; a buckle load calculation section that calculates a buckle load (Wb), which is a load of a side where the buckle is disposed in the portion of the seat, based on the seat loads detected by the load detecting bodies; and a child decision section that prohibits the occupant decision section from changing a decision to a decision result that an adult is seated in the vehicle seat and changes the decision to a decision result that the child seat is attached on the vehicle seat when an engagement of the tongue with the buckle is detected by the seat belt wearing detection body and the buckle load calculated by the buckle load calculation section is less than a predetermined buckling load threshold (Wbth1, Wbth2, Wbth3) in a case where the occupant decision section decides that an adult is seated in the vehicle seat based on the seat loads detected by the load detecting bodies.