1. A biometric information processing apparatus comprising: a calculation unit that calculates a direction of each of characteristic lines included in biometric information extracted from a user; a smoothing unit that smoothes each of the characteristic lines; a difference calculation unit that calculates a difference between the direction of each of the characteristic lines and a direction of each of the smoothed characteristic lines; a detection unit that detects, as a change candidate area, a potentially changeable area in the biometric information based on each of the calculated differences; and an estimation unit that estimates a future authentication accuracy using the biometric information based on the detected change candidate area.