The dc current-voltage (Ⅳ) characteristics of a superconducting single electron transistor irradiated with microwaves up to 18 GHz are experimentally studied. The switching current as a function of gate voltage demonstrates clear phase-charge duality in a Josephson junction. At higher microwave power levels, Shapiro steps in Ⅳ characteristics are observed. The step height in Ⅳ can be analyzed using the model an ac-voltage source applied to a single Josephson junction.