A position calculating system for being mounted on a haulage vehicle including wheels, and a body frame which is mounted on the wheels via suspensions and carries a body mounted thereon, comprising a wheel rotational speed sensor configured to output a rotational speed of one of the wheels and an attitude detection sensor which includes an acceleration sensor and a gyro sensor attachable to the body frame, the attitude detection sensor being configured to output attitude information including acceleration and rotation angular velocity outputted from the acceleration sensor and the gyro sensor, wherein the position calculating system is provided with: a loading status information acquiring unit configured to acquire loading status information on the body frame, said information indicating whether the body frame is in a loaded state with a load carried thereon or in an unloaded state with the load not carried thereon; a stop determination unit configured to determine whether the haulage vehicle is at stop or moving based on outputs from the acceleration sensor and the gyro sensor, or from the wheel rotational speed sensor; a correction amount setting unit that is configured to calculate, as a result of a decision which is made based on the determination by the stop determination unit and the loading status information, when the haulage vehicle is at stop and a status of the haulage vehicle changes from an unloaded state to a loaded state, a correction amount required for bringing x'-axis of attitude information detected in the loaded state into coincidence with x-axis of attitude information detected in the unloaded state, which is in coincidence with a longitudinal direction of motion of the haulage vehicle, bringing y'-axis of the attitude information detected in the loaded state into coincidence with y-axis of the attitude information detected in the unloaded state, which is in coincidence with a lateral direction of the motion of the haulage vehicle, and bringing z'-axis of the attitude information detected in the loaded state into coincidence with z-axis of the attitude information detected in the unloaded state, which perpendicularly intersects the x-axis and the y-axis, and that is configured to set the correction amount as a correction amount for use in calculating a velocity vector of the haulage vehicle in the loaded state; a velocity vector calculating unit that is configured to determine, based on the loading status information, whether the body frame is in the loaded state or the unloaded state and that, when the body frame has been determined to be in the loaded state, to obtain an uncorrected temporary velocity vector having a direction of motion of the haulage vehicle as determined from a wheel speed available from the rotational speed of the one wheel and the attitude information, and to calculate the velocity vector by correcting the temporary velocity vector using the set correction amount, and a position calculating unit that is configured to calculate a position of the haulage vehicle by using the velocity vector. a loading status information acquiring unit configured to acquire loading status information on the body frame, said information indicating whether the body frame is in a loaded state with a load carried thereon or in an unloaded state with the load not carried thereon; a stop determination unit configured to determine whether the haulage vehicle is at stop or moving based on outputs from the acceleration sensor and the gyro sensor, or from the wheel rotational speed sensor; a correction amount setting unit that is configured to calculate, as a result of a decision which is made based on the determination by the stop determination unit and the loading status information, when the haulage vehicle is at stop and a status of the haulage vehicle changes from an unloaded state to a loaded state, a correction amount required for bringing x'-axis of attitude information detected in the loaded state into coincidence with x-axis of attitude information detected in the unloaded state, which is in coincidence with a longitudinal direction of motion of the haulage vehicle, bringing y'-axis of the attitude information detected in the loaded state into coincidence with y-axis of the attitude information detected in the unloaded state, which is in coincidence with a lateral direction of the motion of the haulage vehicle, and bringing z'-axis of the attitude information detected in the loaded state into coincidence with z-axis of the attitude information detected in the unloaded state, which perpendicularly intersects the x-axis and the y-axis, and that is configured to set the correction amount as a correction amount for use in calculating a velocity vector of the haulage vehicle in the loaded state; a velocity vector calculating unit that is configured to determine, based on the loading status information, whether the body frame is in the loaded state or the unloaded state and that, when the body frame has been determined to be in the loaded state, to obtain an uncorrected temporary velocity vector having a direction of motion of the haulage vehicle as determined from a wheel speed available from the rotational speed of the one wheel and the attitude information, and to calculate the velocity vector by correcting the temporary velocity vector using the set correction amount, and a position calculating unit that is configured to calculate a position of the haulage vehicle by using the velocity vector.