A calibration system is mounted in a vehicle and is provided with: an attitude sensor that acquires attitude information regarding a vehicle coordinate system of the vehicle; an image pickup unit that captures an image of the surroundings of the vehicle; an attitude-sensor reliability level calculation unit that calculates an attitude-sensor reliability level that is a reliability level of the attitude sensor; and an attitude use estimation unit that estimates, in the case where the calculated attitude-sensor reliability level is greater than a predetermined threshold, an external parameter of the image pickup unit using the attitude information output by the attitude sensor and a relationship of a coordinate system of the image pickup unit to the vehicle coordinate system.