An impedance matching device includes: a variable capacitor; a calculation unit that calculates a reflection coefficient on the load side; a storage unit that stores the reflection coefficient calculated within a predetermined period so as to be associated with ON/OFF states of the semiconductor switches; a determination unit that determines ON/OFF states to be taken by the semiconductor switches using a calculation result within the predetermined period; a control unit that turns on or off the semiconductor switches based on the determined ON/OFF states; and a counting unit that counts the number of times the determined ON/OFF states have changed. In a case where the counted number of times is larger than a predetermined number of times, the control unit turns on or off the semiconductor switches so as to match ON/OFF states associated with a reflection coefficient closer to 0, among the stored reflection coefficients, and then prohibits ON/OFF switching.