Hydrothermal treatment was investigated as a new pretreatment method for biodegradability improvement of wastewater containing vegetable proteins. Soybean milk was used as a model wastewater. After 7.4 min of hydrothermal treatment, HOD, of a sample treated under the reaction conditions of 300degreesC and 9 MPa was twice as high as that of raw sample. Moreover the D-BOD5/D-CODCr ratio of the treated sample reached 0.80, while that of the raw sample was 0.54. The results of GPC analysis indicated that the biodegradability improvement of the treated sample was due to a shift of the molecular weight distribution to a low molecular weight range. Next, tofu industrial wastewater, an actual wastewater, was tested under the reaction conditions of 300degreesC and 9 MPa that were optimal for the treatment of soybean milk. The ratios of D-BOD5/D-CODCr of treated sample and raw sample were 0.70 and 0.49, respectively. Thus, hydrothermal treatment can be a useful pretreatment to improve biodegradability of wastewater before a conventional biological treatment process.