A blood sugar condition is estimated by measuring a urine sugar value, discriminating whether it is less than a discriminating method classification boundary value, when it is less, obtaining an averaged value of measured urine sugar values during a past predetermined period of time, measured at the same time of day as the latest measurement, calculating the difference between the averaged and measured urine sugar values, when the difference is not less than a positive discriminating reference difference, discriminating as being a blood sugar value decrease, when the measured value is not less than the discriminating method classification boundary value, obtaining an averaged value of all measured values during the past predetermined period of time, calculating a ratio between the measured and averaged values, and, when the ratio is not greater than a first discriminating reference ratio, discriminating as being the blood sugar value decrease.