A biological information monitor comprising: a first measuring unit which is configured to measure a pulse wave propagation time of a patient; a second measuring unit which is configured to measure a blood pressure of the patient; a calculating unit which is configured to calculate an estimated blood pressure value of the patient based on the pulse wave propagation time of the patient; a setting unit (10, 11) which is configured to set a threshold, the threshold including first and second upper limits (a1, a2) which are different from each other and first and second lower limits (b1, b2) which are different from each other; and a determining unit which configured to compare the estimated blood pressure value with the threshold to obtain a comparison result, wherein the second measuring unit is activated to measure the blood pressure of the patient at least one of at time intervals and at a time when an operator operates the second measuring unit, and the second measuring unit is activated to measure the blood pressure of the patient by the determining unit based on the comparison result; wherein the first upper limit is upper than the second upper limit; when the estimated blood pressure value exceeds the first upper limit, the second measuring unit is activated to measure the blood pressure of the patient, and after the estimated blood pressure value exceeds the first upper limit, the second measuring unit is not activated unless the estimated blood pressure value falls below the second upper limit and then exceeds the first upper limit again; wherein the first lower limit is lower than the second lower limit; when the estimated blood pressure value falls below the first lower limit, the second measuring unit is activated to measure the blood pressure of the patient, and after the estimated blood pressure value falls below the first lower limit, the second measuring unit is not activated unless the estimated blood pressure value exceeds the second lower limit and then falls below the first lower limit again. a first measuring unit which is configured to measure a pulse wave propagation time of a patient; a second measuring unit which is configured to measure a blood pressure of the patient; a calculating unit which is configured to calculate an estimated blood pressure value of the patient based on the pulse wave propagation time of the patient; a setting unit (10, 11) which is configured to set a threshold, the threshold including first and second upper limits (a1, a2) which are different from each other and first and second lower limits (b1, b2) which are different from each other; and a determining unit which configured to compare the estimated blood pressure value with the threshold to obtain a comparison result, wherein the second measuring unit is activated to measure the blood pressure of the patient at least one of at time intervals and at a time when an operator operates the second measuring unit, and the second measuring unit is activated to measure the blood pressure of the patient by the determining unit based on the comparison result; wherein the first upper limit is upper than the second upper limit; when the estimated blood pressure value exceeds the first upper limit, the second measuring unit is activated to measure the blood pressure of the patient, and after the estimated blood pressure value exceeds the first upper limit, the second measuring unit is not activated unless the estimated blood pressure value falls below the second upper limit and then exceeds the first upper limit again; wherein the first lower limit is lower than the second lower limit; when the estimated blood pressure value falls below the first lower limit, the second measuring unit is activated to measure the blood pressure of the patient, and after the estimated blood pressure value falls below the first lower limit, the second measuring unit is not activated unless the estimated blood pressure value exceeds the second lower limit and then falls below the first lower limit again.