This report describes an ingestible capsule sensor that can wirelessly detect the activity of gut bacteria by irradiating microwaves from outside the body. The proposed sensor is composed of only edible materials: the critical sensing material is a dietary fiber, guar gum, that is decomposed by gut bacteria. We confirmed that the guar gum film can be decomposed by gut bacteria, bifidobacteria. For wireless communication, we fabricated an ingestible antenna made from rice paper and gold with a split-ring resonator (SRR), one of the elements of electromagnetic metamaterials. Our sensor fulfills swallowable size and wireless communication requirements for the first time and would be a safety sensor for gastrointestinal (GI) tract monitoring. We believe that our ingestible sensor could be an effective tool for non-invasive biological substances monitoring without retention risk.