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 5 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 β, which indicate a posture of a portion to which the wearable device is attached, are 10calculated based on the accelerations (Bx By, Bz) obtained by the conversion based on the correction angle γ.
