To provide a sensing device having a high processing power and capable of high-accuracy measurement. It is determined whether or not an oscillation frequency is stabilized while a buffer solution is supplied to a quartz-crystal resonator 4. from a syringe pump 10. When it is determined that the frequency is stabilized, a second valve 14. is switched to a sample solution supply mode to supply a sample solution in an injection loop 14a. to the quartz-crystal resonator 4. An instant at which the sample solution reaches the quartz-crystal resonator 4. and an instant at which the sample solution finishes passing through the quartz-crystal resonator 4. are automatically found based on a supply flow rate of the buffer solution, a volume of the injection loop 14a, a volume of a supply channel supplying the sample solution to the quartz-crystal resonator 4, and an instant at which the second valve 14. is switched to the sample solution supply mode. An oscillation frequency before the sample solution reaches the quartz-crystal resonator 4. and an oscillation frequency after the sample solution passes through the quartz-crystal resonator 4. are found, and a difference between the oscillation frequencies is obtained.