An ingestible sensor according to an embodiment includes a sensor, a detector, and a transmitter and to be mixed with food and discharged without being digested or absorbed also when entering the inside of a body. The sensor is configured to detect a predetermined substance disposed inside the body. The detector is configured to detect whether or not the sensor has entered the inside of the body. The transmitter is configured to transmit information of the predetermined substance detected by the sensor to a communication device disposed outside the body based on a detection of an entrance of the sensor into the inside of the body that is made by the detector.
