To provide a compact controller of steam quality capable of precisely controlling wetness and the like without using a wetness detection means.

A pressure control valve 2, a gas-liquid separator 3, a pressure sensor 4, a vapor flowmeter 5, a heating/cooling means 6 and a temperature sensor 7 are attached to a steam pipe 1. A high pressure steam pipe 13 and a cooling water supply pipe 14 are connected to the heating/cooling means 6. Automatic control valves 17, 16 are respectively provided to pipes 13, 14. Steam flowing down the steam pipe 1 is separated into condensate and steam by the gas liquid separator 3 and only the steam flows into the steam flowmeter 5. From a flow rate detection value, a pressure value and a temperature value by the steam flowmeter 5, required heating quantity or required cooling quantity by the heating/cooling means 6 is calculated and steam is made to have an intended wetness, dryness or degree of superheat by conducting predetermined heating or cooling to the steam in the steam pipe 1.