An elevator system controller for efficient group supervisory control while avoiding collisions between two elevator cars in service in a single elevator shaft. The elevator system controller includes a risk calculating unit for calculating a risk of a collision between elevator cars in the same shaft when the elevator cars are responding to a new call for service, a car assigning unit for assignment of an elevator car to respond to the new call based on the risk of collision, and an operation control unit for controlling operation of the elevator cars based on the assignment by the car assigning unit. The risk of collision is calculated for each car, and the risk is recalculated based on a possibility of a withdrawal of one of the elevator cars to a position in the shaft where no collision can occur, based on a predicted arrival time of a car at the floor requiring service. Cars that have high risks of collision when the remaining cars in the same shaft cannot be withdrawn in time to a safe spot are removed as candidates for assignment to respond to the new call. An evaluation is carried out using several evaluation indexes, in addition to the risk of collision, to decide which car is to be assigned to respond to the new call. If a determination of a traffic condition indicates low usage of the elevator cars, one car in each shaft is forwarded to a rest position and paused.