An in vivo information acquiring apparatus (10E, 10F) comprising: an in vivo information acquiring section that acquires information on an inside of a body of a subject; a power source that supplies power; a power source switch that turns on/off power supply from the power source to the in vivo information acquiring section; a detection section that upon detection of introduction to the inside of the body of the subject outputs a detection signal; and a control section that controls the power source switch according to the detection signal; characterized in that the detection section includes at least two electrodes (20A, 20B) disposed on an outer face of a casing, and outputs the detection signal upon detection of introduction to the inside of the body of the subject based on a change in electric resistance between the electrodes (20A, 20B); the in vivo information acquiring apparatus (10E, 10F) further comprises a signal receiving section (11E, 11 F) that receives a control signal from an outside and outputs an internal signal, the control signal being a magnetic field control signal; and the control section performs on/off control of the power source switch according to the internal signal in a state where the detection section does not output the detection signal wherein in said state the power source switch toggles on/off power supply to the in vivo information acquiring section each time a magnetic field is applied, and controls the power source switch to be turned on according to the detection signal, irrespective of the internal signal, in a state where the detection section outputs the detection signal. an in vivo information acquiring section that acquires information on an inside of a body of a subject; a power source that supplies power; a power source switch that turns on/off power supply from the power source to the in vivo information acquiring section; a detection section that upon detection of introduction to the inside of the body of the subject outputs a detection signal; and a control section that controls the power source switch according to the detection signal; characterized in that the detection section includes at least two electrodes (20A, 20B) disposed on an outer face of a casing, and outputs the detection signal upon detection of introduction to the inside of the body of the subject based on a change in electric resistance between the electrodes (20A, 20B); the in vivo information acquiring apparatus (10E, 10F) further comprises a signal receiving section (11E, 11 F) that receives a control signal from an outside and outputs an internal signal, the control signal being a magnetic field control signal; and the control section performs on/off control of the power source switch according to the internal signal in a state where the detection section does not output the detection signal wherein in said state the power source switch toggles on/off power supply to the in vivo information acquiring section each time a magnetic field is applied, and controls the power source switch to be turned on according to the detection signal, irrespective of the internal signal, in a state where the detection section outputs the detection signal.