A portable terminal apparatus is mounted in a casing and has a magnetic sensor that detects a magnetic field around the casing with an offset and outputs magnetic data representing the detected magnetic field, and a tilt sensor that detects a tilt of the casing and sequentially outputs tilt data representing the detected tilt. In the portable terminal apparatus, a capture part sequentially captures the magnetic data from the magnetic sensor at a given time interval. A calculation part calculates the offset of the magnetic sensor based on a plurality of the magnetic data sequentially captured by the capture part. A detection part detects an orientation variation of the casing based on the tilt data sequentially outputted from the tilt sensor. A change part changes the time interval for sequentially capturing the magnetic data in accordance with the detected orientation variation.