A first acceleration sensor is arranged on a crank mounted to a crankshaft so as to be located at a position away from the crankshaft by a distance r1, and the first acceleration sensor detects acceleration a1 in a direction parallel to a longitudinal direction of the crank. A second acceleration sensor is arranged on the crank mounted to the crankshaft so as to be located at a position away from the crankshaft by a distance r2 different from the distance r1, and the second acceleration sensor detects acceleration a2 in a direction parallel to the longitudinal direction of the crank. A measurement module control part calculates information regarding a rotation angle of the crank based on a1, a2, r1, and r2.