A blood volume measuring method includes: acquiring data of stroke volume variation at each respiratory rate, and storing the acquired data in a first buffer reading an N1 number of the data stored in the first buffer, from the first buffer, and storing the N1 number of the data in a second buffer excluding a part of the data stored in the second buffer, a value of each of which exceeds a first upper limit or falls below a first lower limit, from the data stored in the second buffer, and storing the other of the data stored in the second buffer, in a third buffer acquiring a deviation of each of the data stored in the third buffer, from a median value of the data stored in the third buffer, excluding a part of the data stored in the third buffer, the deviation of each of which exceeds a second upper limit or falls below a second lower limit, from the data stored in the third buffer, and storing the other of the data stored in the third buffer, in a fourth buffer determining whether N2/N1 is within an allowable value or not, wherein the N2 is a number of the data stored in the fourth buffer and when it is determined that the N2/N1 is within the allowable value, calculating data of average stroke volume variation based on the data stored in the fourth buffer.
