To provide an image data transmission buoy, capable of monitoring the state of the water level and the water quality based on the data of the same at a remote place.

The image data transmission buoy 50 is provided with a main body part 4, a float part 13 to provide buoyancy to the main body part 4, an upper camera (image pick-up means) 2 and a lower camera (image pick-up means) 9 provided at an upper and a lower proper position of the main body part 4, a satellite transmitter/receiver (transmission/receiving part) 12 to transmit data processed of picked up images by the upper camera 2 and the lower camera 9, and receive instruction data from the external by radio communication, a movable means 15 to move the main body part 4, a control part 6 to control the upper camera 2, the lower camera 9, the movable means 15, and the satellite transmitter/receiver 12, and a battery (power source part) 7 to supply power to the upper camera 2, the lower camera 9, the satellite transmitter/receiver 12, the movable means 15, and the control part 6.