We have investigated coherent properties of an artificial two-level system realized in a small-Josephson-junction circuit. Two charge-number states of a small superconducting electrode connected to a reservoir via a Josephson junction were used as the two relevant states. Coherent manipulations of the charge-number states by applying a gate-voltage pulse were demonstrated, and decoherence of the system was studied through a spin-echo-type experiment. The result suggested that low-frequency energy-level fluctuations due to 1/f background charge noise is a dominant source of the dephasing.