A protection circuit 26c detects a voltage at a point A on an electric-power supply path from a battery 29 to an intra-capsule function executing circuit 30. When the voltage at the point A becomes smaller than a predetermined threshold value (a predetermined midpoint potential), the protection circuit 26c performs a switching control of a switch element 26a, connects the battery 29 to a resistive load 26b, stops supplying driving power to the intra-capsule function executing circuit 30 and at the same time enables to supply electric power to the resistive load 26b, and prevents a false operation of a circuit at the midpoint potential by exhausting the electric power stored in the battery 29 by the resistive load 26b.