Information (particularly, water content) of a sheet material which changes during progression of processing is estimated with higher accuracy and information output or (image formation) processing is performed. The water content estimation method of the present invention includes: detecting a first information regarding moisture contained in a sheet material; detecting a second information regarding a factor affecting information regarding the moisture contained in the sheet material; predicatively computing an estimation of a water content of the sheet material based on the first information and the second information; and adjusting processing conditions for image formation based on the estimation of the water content which is computed.