PURPOSE: To enable data interpolation with less effect on the analysis of electrocardiographic waves.

CONSTITUTION: At the step S1, it is judged whether a sampling data of an electrocardiographic wave is an error or not and when it is determined to be the error at the step S1, it is judged at the step S8 whether an effective data exists or not in seven samples ahead. When no effective data exists in the seven samples ahead, a preholding is performed at the step 9. When any effective data exists in the seven samples ahead, a linear interpolation is performed at the steps 4 and 5.