1. A physical quantity measuring device for measuring a physical quantity, comprising: a vector physical quantity detection means for detecting a vector physical quantity composed of a plurality of components; a datum acquisition means for repeatedly obtaining the detected vector physical quantity as vector physical quantity datum to obtain vector physical quantity data; and an offset estimation means for calculating difference vectors from the obtained vector physical quantity data and statistically estimating an offset included in the obtained vector physical quantity data based on a predetermined evaluation formula using the calculated difference vectors, wherein the offset estimation means includes: a difference vector calculating portion for calculating the difference vectors using a difference between each component of the obtained vector physical quantity data; a data selection portion that selects and stores the difference vectors based on a predetermined measurement parameter; a reference point estimation portion that statistically estimates, based on the evaluation formula using the selected difference vectors, coordinates of a reference point on a coordinate system where each component of the obtained vector physical quantity data is a coordinate value; and a reliability determination portion that calculates reliability information on the reference point using the difference vectors according to a calculation parameter for calculating the reliability information on the reference point, that compares the reliability information with a determination threshold value to determine a degree of reliability on the reference point and that outputs the reference point determined to have a desired reliability as an offset included in the vector physical quantity data obtained by the datum acquisition means; and wherein the evaluation formula is determined using an N-th power of an absolute value of an inner product of the difference vector and a vector connecting a middle point of the difference vector and the reference point.