To provide safety-ensuring information that is considered to enhance safety in accordance with the movement of the diver's body in the water. An information processing device is provided that is worn on the diver, and has an external sensor unit 5 to measure environment information around the wearing location and to transmit environment information data, and a dive computer 4 to receive environment information data from the external sensor unit 5 and to generate and to output safety-ensuring information to ensure the safety of the diver on the basis of the environment information corresponding to the environment information data.