1. A posture information calculation device that calculates a posture angle and position coordinates in a virtual three-dimensional space based on three-axis angular velocity information detected by three angular velocity sensors, and three-axis acceleration information detected by three acceleration sensors, the three-axis angular velocity information being information about angular velocities around three axes that are not parallel to one another, and the three-axis acceleration information being information about accelerations in three axial directions that are not parallel to one another, the posture information calculation device comprising: a sensor information acquisition section that acquires the three-axis angular velocity information, and the three-axis acceleration information; and a posture information calculation section that calculates the posture angle and the position coordinates in the virtual three-dimensional space based on the three-axis angular velocity information and the three-axis acceleration information acquired by the sensor information acquisition section, the posture information calculation section calculating a fixed coordinate system velocity vector based on an inertial coordinate system acceleration vector obtained from the three-axis acceleration information, and calculating the position coordinates in the virtual three-dimensional space corresponding to the fixed coordinate system velocity vector, the posture information calculation section including: a posture information update section that updates a parameter necessary for specifying the posture angle and the position coordinates in the virtual three-dimensional space based on the three-axis angular velocity information and the three-axis acceleration information; a posture information correction section that corrects the parameter updated by the posture information update section under a constraint condition; and a posture information generation section that calculates the posture angle and the position coordinates in the virtual three-dimensional space based on the parameter corrected by the posture information correction section, the posture information update section including a velocity vector update section that updates the fixed coordinate system velocity vector at a time k based on the inertial coordinate system acceleration vector and the fixed coordinate system velocity vector at a time k−1; the posture information correction section including a velocity vector correction section that corrects the fixed coordinate system velocity vector at the time k updated by the velocity vector update section under a constraint condition that a norm of the fixed coordinate system velocity vector is zero; the posture information generation section including a position coordinate generation section that generates the position coordinates in the virtual three-dimensional space corresponding to the fixed coordinate system velocity vector at the time k corrected by the velocity vector correction section; and the sensor information acquisition section and the posture information calculation section being implemented by a processor, the three angular velocity sensors and the three acceleration sensors.