A sensitivity switchable detection circuit includes a high gain circuit for outputting a first signal, a low gain circuit for outputting a second signal, and an output switching circuit for switching between the first signal and the second signal. When the first signal is smaller than a lower limit, the first signal determines a detected signal. When the second signal is larger than an upper limit, the second signal determines the detected signal. When the first signal is larger than the lower limit and the second signal is lower than the upper limit, a weighting function that uses the first signal and the second signal as input variables determines the detected signal.