A calibration system mounted in a vehicle and comprising: 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 first 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, characterized by, a camera reliability level calculation unit that calculates a camera reliability level that is a reliability level of the image pickup unit; an image use estimation unit that estimates, in the case where the calculated camera reliability level is greater than a second predetermined threshold, an external parameter of the image pickup unit using output of the image pickup unit; and a relative attitude calculation unit that calculates, in the case where the calculated attitude-sensor reliability level is greater than the first predetermined threshold and the calculated camera reliability level is greater than the second predetermined threshold, a relationship of a coordinate system of the image pickup unit to the vehicle coordinate system, based on the external parameter estimated by the image use estimation unit and the attitude information output by the attitude sensor, and a running state determination unit that is capable of determining a straight-traveling state and a turning state of the vehicle; and an attitude sensor correction unit that calibrates the attitude sensor based on output of the attitude sensor and output of the image pickup unit in the straight-traveling state determined by the running state determination unit and output of the attitude sensor and output of the image pickup unit in the turning state determined by the running state determination unit, wherein the attitude-sensor reliability level calculation unit calculates the attitude-sensor reliability level based on a period of time elapsed from the time when the calibration was performed by the attitude sensor correction unit. 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 first 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, characterized by, a camera reliability level calculation unit that calculates a camera reliability level that is a reliability level of the image pickup unit; an image use estimation unit that estimates, in the case where the calculated camera reliability level is greater than a second predetermined threshold, an external parameter of the image pickup unit using output of the image pickup unit; and a relative attitude calculation unit that calculates, in the case where the calculated attitude-sensor reliability level is greater than the first predetermined threshold and the calculated camera reliability level is greater than the second predetermined threshold, a relationship of a coordinate system of the image pickup unit to the vehicle coordinate system, based on the external parameter estimated by the image use estimation unit and the attitude information output by the attitude sensor, and a running state determination unit that is capable of determining a straight-traveling state and a turning state of the vehicle; and an attitude sensor correction unit that calibrates the attitude sensor based on output of the attitude sensor and output of the image pickup unit in the straight-traveling state determined by the running state determination unit and output of the attitude sensor and output of the image pickup unit in the turning state determined by the running state determination unit, wherein the attitude-sensor reliability level calculation unit calculates the attitude-sensor reliability level based on a period of time elapsed from the time when the calibration was performed by the attitude sensor correction unit.