The K-Ar ages and paleomagnetic stratigraphy of the Plio-Pleistocene volcanic rocks from the Ariake Sea region have been studied to clarify the evolution of volcanism in the western part of Beppu-Shimabara graben, west Kyushu, Japan. This region has seven domains of volcanic rocks. We determined K-Ar ages of ten volcanic rocks from five domains: the Misumi volcanic rocks=4.51 Ma and 3.97 Ma; the Kunisaki andesite=1.45 Ma; the Ibinoodake andesite=3.97 Ma and 4.52 Ma; the Uki volcanic rocks=1.63 Ma; the Yushima basalt=0.82 Ma; the Otake volcanic rocks=1.82, 1.77, and 1.72 Ma. Paleomagnetic measurements were made on volcanic rocks collected from all seven domains. Magnetic property of lavas from these domains can be classified into four magnetic polarity epochs: Brunhes Chron (Ninotake volcano, Takesaki basalt and Yushima basalt), Gauss Chron (Nogamajima volcano), Gilbert Chron (Ibinoodake andesite and Hidake lava dome), and Matsuyama Chron (the other volcano). The age of Ohya tuff, which belongs to the Kuchinotsu Formation, corresponds to Gilsa event. We synthesized our new data together with previously reported ages and stratigraphic data, and examined the volcanic activity in this region in terms of a spatial and temporal distribution. There is no correlation between spatial distribution of the volcanic rocks and eruption ages, but we revealed that the volcanic activity in this region is episodic during the last 5 million years. We can identify the magmatic activities as five episodes: 5-4 Ma, 3.2 Ma, 2.0-1.4 Ma, 1.0 Ma, and <0.9 Ma. The intensity of volcanic activity in the Ariake Sea region increased with time.