to st2 since the average pulse wave ratio r n remains smaller than the 40 level however when the cuff is sufficiently depressurized and the average pulse wave ratio value r n exceeds the 40 level the determination result of st10 changes to yes and the steps from st11 onward are carried out if the variable r max is found to be smaller than the average pulse wave ratio r n in st11 the cuff pressure value is stored in the variable dia in st16 and the count c dia is set to zero st17 further the current average pulse wave ratio value r n is stored in r max st18 and the program flow returns to st2 thereafter the steps in st2 through st11 st16 through st18 are repeated in other words the variable dia is updated by the cuff pressure until the average pulse wave ratio r n reaches a maximum value if the average pulse wave ratio r n is found to be smaller than variable r max in st11 the program flow advances to st12 where the counter c dia is incremented the count c dia corresponds to the number of pulses after the average pulse wave ratio r n passed its maximum level or.