To provide a printing apparatus which has a treatment device for applying treatment liquid to a printing plate on a plate cylinder and solves problems such as a development shortage caused by the lowering of the temperature of the treatment liquid and a method for treating the printing plate in the printing apparatus.

The printing apparatus has a development tank 20 for storing development liquid, a development tank 28 for storing the development liquid, a temperature sensor 30 for measuring the temperature of the tank 28, a heater 31 for heating the development liquid in the tank 28, and a temperature sensor 34 for measuring the temperature of the periphery of the plate cylinder. On the basis of the temperature tw of the periphery of the plate cylinder measured by the temperature sensor 34, the conditioning temperature of the treatment liquid is set from a look-up table(LUT) stored in advance, and the temperature of the temperature T development liquid is controlled.