A crystal oscillator (31) (Y-cut) has a temperature characteristic in which its oscillating frequency significantly changes with temperature, whereas a crystal oscillator (32) (AT-cut) has a temperature characteristic in which its oscillating frequency is stable with temperature. Crystal oscillators (31, 32) are cut from a raw material of the same type and configured to be substantially equal in shape, material, and size, and provide a combination of oscillation frequencies such that the frequency of a signal generated by a differential frequency circuit (35) will be less than or equal to 10 kHz within a measuring temperature range of 21 to 30° C. The frequency of a signal generated by differential frequency generating circuit (35) is output to a measurement apparatus main unit and a frequency counting circuit (15) measures the frequency of this signal by a reciprocal counting method to obtain at least eight or more significant digits.