An updating unit updates an attitude represented by quaternion by defining an initial attitude represented by quaternion as an initial value, and successively substituting output values of the triaxial gyro sensor. A second converter converts the attitude represented by quaternion into an attitude in the Euler angle representation. An angular speed derivation unit derives an angular speed based on a time-dependent change in the attitude in the Euler angle representation. The output unit outputs an angular speed derived based on an output value of a monoaxial gyro sensor, when a difference between a pitch angle derived based on the output value of the triaxial acceleration sensor and the output value of the second converter and a pitch angle from the second converter is equal to or larger than a threshold value.
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