The invention provides a blood purification device and a method of controlling the blood purification device, which can raise visual identity of a display part representing a relationship between a measuring pressure value and a pressure alarm value in a display unit of the blood purification device. The blood purification device comprises a blood loop for performing blood purification processing, a pressure testing unit for testing the pressure of the blood loop, a setting unit for setting a pressure alarm value specific to measuring pressure of the pressure testing unit, a display unit for displaying the relationship between the measuring pressure value measured by the pressure measuring unit and the pressure alarm value set in the setting unit, and a pressure range changing unit for continuously changing the pressure value range displayed in the display unit and the pressure alarm value set in the setting unit to make an upper limit value of the pressure value which can be displayed by the display unit be a greatest value of the pressure alarm value or be a value obtained by adding the largest value and a prescribed value. A lower limit value of the pressure value which can be displayed by the display unit is a smallest value of the pressure alarm value or be a value obtained by subtracting a prescribed value from the smallest value.
