  By using the Schwinger-Keldysh approach, we evaluate the current noise and
the charge noise of the single-electron transistor (SET) in the regime of large
charge fluctuations caused by large tunneling conductance. Our result
interpolates between previous theories; the "orthodox" theory and the
"co-tunneling theory". We find that the life-time broadening effect suppresses
the Fano factor below the value estimated by the previous theories. We also
show that the large tunnel conductance does not reduce the energy sensitivity
so much. Our results demonstrate quantitatively that SET electrometer can be
used as the high-sensitivity and high-speed device for quantum measurements.
