  We propose a new method to measure the system noise temperature, $T_{\rm
sys}$, using a 2-bit analog-to-digital converter (ADC). The statistics of the
digitized signal in a four-level quantization brings us information about the
bias voltage and the variance, which reflects the power of the input signal.
Comparison of the variances in {\it hot} and {\it sky} circumstances yields
$T_{\rm sys}$ without a power meter. We performed test experiments using the
Kagoshima 6-m radio telescope and a 2-bit ADC to verify this method. Linearity
in the power-variance relation was better than 99% within the dynamic range of
10 dB. Digitally measured $T_{\rm sys}$ coincided with that of conventional
measurement with a power meter in 1.8-% difference or less for elevations of
$10^{\circ} - 88^{\circ}$. No significant impact was found by the bias voltages
within the range between -3.7 and +12.8% with respect to the threshold voltage.
The proposed method is available for existing interferometers that have a
multi-level ADC, and release us from troubles caused by power meters.
