A peritoneal function testing method uses a ratio of MTACun/MTACc, calculated using MTACun and MTACc, as an index for a peritoneal function test, wherein MTACun is an overall mass transfer-area coefficient for urea nitrogen and MTACun is an overall mass transfer-area coefficient for creatinine. The use of MTACun/MTACc enables examination of the future peritoneal function of a patient (a mechanism of deterioration in peritoneal function). To be specific, MTACun and MTACc can be obtained by computing a Pyle-Popovich model. In addition, the peritoneal function testing method further calculates a permeability coefficient for cell pores (LpSC) and an overall permeability coefficient (LPS) from a Three-Pore Theory model while obtaining a ratio of LPSC/LPS calculated using the LPSC and the LPS. The LPSC/LPS ratio and the MTACun/MTACc ratio can be used as indexes for the peritoneal function test.