Metal pollution and metal shortage are a growing threat to the global environment and world high-tech industry. One of the promising strategies for removing and recycling metal-elements from the environments is applying Metal-Biotechnology based on metal-related biological activities, which include bioaccumulation, bioadsorption, and biomineralization. In this study, focusing on Manganese (Mn) pollution, we have isolated and analyzed the Mn(II)-oxidizing marine bacterium, Arthrobacter sp. NI-2 strain, from Imari Bay, Imari-shi, Saga, Japan. We have also isolated a spontaneous mutant, Arthrobacter sp. NI-2' strain with enhanced Mn(II)-oxidizing activity. Under the liquid culture condition at 30 °C, Arthrobacter sp. NI-2' strain could efficiently remove more than 96% of Mn(II) from the liquid culture media containing 0.4 mM Mn(II). Although Mn(II)-oxidizing activity of Arthrobacter sp. NI-2 strain is suppressed under the low temperature conditions, the increased Mn(II)-oxidizing activity of Arthrobacter sp. NI-2' strain was maintained when the growth temperature was shifted from 30 °C to 10 °C. Therefore, the Arthrobacter sp. NI-2' strain would be useful as a tool for Mn removal from low-temperature water, such as groundwater around the mining area.