Swing measurement device, swing measurement method, and swing measurement program

A swing measurement system measures an evaluation index of a swing of a golf club using an inertial sensor. A computer is provided with: a movement trajectory calculation unit for calculating a movement trajectory of the golf club during the swing using a detection value of the inertial sensor; a shaft plane calculation unit for calculating a shaft plane of the swing based on the movement trajectory in a freely selectable section of the swing; and an angle calculation unit for calculating, as the evaluation index, a shaft plane angle formed between the face surface of the golf club and the shaft plane.