A sensing device for measuring the concentration of an object to be sensed by using a crystal sensor or judging whether or not the object to be sensed exists. The sensing device has a structure in which an oscillation circuit unit and a device body containing a measuring section are separated, and the device body enables the user to grasp the oscillation frequency of the connected oscillation circuit unit. Bandpass filters having pass characteristics corresponding to the respective oscillation frequencies of the oscillation circuit are provided in the device body to discriminate the frequency signal. Whether the level of the discriminated frequency signal is the threshold value or more is grasped, a switch section is controlled to connect the channel the level of which is the threshold value or more to the measuring section, and a frequency e.g., a reference frequency used by the measuring section is selected according to the frequency of the frequency signal corresponding to the channel.
