A power generating machine having a frame provided with a first support, a second support and a third support laterally and adjustably mounted between the first and second supports. A bellows is mounted under the third support and between the first and second supports. The bellows has a buoy mounted at a bottom of the bellows to adapt to be flush with sea level so that sea waves are able to drive the bellows to move up and down reciprocally. A first tube is mounted to communicate with the first tube and so as to allow air to be sucked in the first tube when the bellows extend. A second tube is communicated with the first tube so as to allow air flowing only to a tank connected to and communicating with the second tube to drive a pneumatic generator to generate electricity.