An air mattress for a bed, placed on a plurality of bottoms including a back bottom and used for the bed that performs a back raising operation for raising the back bottom, including air cells extending to a first direction and being arranged in parallel in a second direction； a first unit capable of inflating and deflating each of the air cells； and, a controller configured to control to alternately inflate and deflate the air cells by successively increasing and decreasing a pressure of air to be supplied to each of the air cells. The first unit inflate and deflate the air cells by n lines independent from each other, where n is equal to or greater than 3, and the controller controls inflation and deflation of air cells of one line during at least a part of a period in which a back bottom is being raised.
