Accelerations (Ax, Ay, Az) in directions along (X′, Y′, Z′) detected by an acceleration sensor 10 are converted, based on a correction angle γ, which is an angle between a first detection axis X′ and a first posture axis X, into accelerations (Bx By, Bz) in directions along three posture axes (X, Y, Z). A pitch angle α and a roll angle ß, which indicate a posture of a portion to which the wearable device is attached, are calculated based on the accelerations (Bx By, Bz) obtained by the conversion based on the correction angle γ.
