In an automatic matching unit, a controller includes a first and a second matching algorithm. The operating point Zp is moved stepwise toward the matching point Zs with a relatively large pitch by using the first matching algorithm. Further, when the operating point Zp is within the outer proximity range, the operating point Zp is moved stepwise toward the matching point Zs with a relatively small pitch by using the second matching algorithm. In the second matching algorithm, the operating point Zp is moved close to the third reference line TC1S or TC2S perpendicular to the first or second reference line C1S or C2S along, e.g., the route Zp(7)→Zp(8)→Zp(9) on the impedance coordinates. The coordinates of the operating point Zp(9) reaches an available quasi-matching point ZB extremely close to the origin O (the matching point Zs).