A wire electrical discharge machine for performing electrical discharge machining on a workpiece by generating electrical discharge at an electrode gap formed between the workpiece and a wire electrode, comprising: a pulse detection unit configured to detect voltage pulses repeatedly applied between the workpiece and the wire electrode; a pulse information acquisition unit configured to determine which type of pulse the voltage pulses detected by the pulse detection unit belong to; an instability calculation unit configured to calculate a degree of instability indicating how unstable a discharge state is; and a machining condition changing unit configured to change a machining condition for the workpiece, based on the calculated degree of instability, characterised in that the pulse information acquisition unit is configured to: determine that the voltage pulses are normal discharge pulses, in a case where a discharge delay time (Ta), which is a time from start of application of the voltage until occurrence of voltage drop due to the electrical discharge, is equal to or longer than a predetermined time, determine that the voltage pulses are abnormal discharge pulses, in a case where the discharge delay time is less than the predetermined time, and determine that the voltage pulses are non-discharge pulses, in a case where the voltage pulses have no voltage drop due to the electrical discharge and the voltage is applied between the workpiece and the wire electrode, wherein the instability calculation unit is configured to calculate the degree of instability, by using a ratio of total time of the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time, to total time obtained by summing the discharge delay time of the normal discharge pulses, the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time. a pulse detection unit configured to detect voltage pulses repeatedly applied between the workpiece and the wire electrode; a pulse information acquisition unit configured to determine which type of pulse the voltage pulses detected by the pulse detection unit belong to; an instability calculation unit configured to calculate a degree of instability indicating how unstable a discharge state is; and a machining condition changing unit configured to change a machining condition for the workpiece, based on the calculated degree of instability, characterised in that the pulse information acquisition unit is configured to: determine that the voltage pulses are normal discharge pulses, in a case where a discharge delay time (Ta), which is a time from start of application of the voltage until occurrence of voltage drop due to the electrical discharge, is equal to or longer than a predetermined time, determine that the voltage pulses are abnormal discharge pulses, in a case where the discharge delay time is less than the predetermined time, and determine that the voltage pulses are non-discharge pulses, in a case where the voltage pulses have no voltage drop due to the electrical discharge and the voltage is applied between the workpiece and the wire electrode, wherein the instability calculation unit is configured to calculate the degree of instability, by using a ratio of total time of the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time, to total time obtained by summing the discharge delay time of the normal discharge pulses, the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time. determine that the voltage pulses are normal discharge pulses, in a case where a discharge delay time (Ta), which is a time from start of application of the voltage until occurrence of voltage drop due to the electrical discharge, is equal to or longer than a predetermined time, determine that the voltage pulses are abnormal discharge pulses, in a case where the discharge delay time is less than the predetermined time, and determine that the voltage pulses are non-discharge pulses, in a case where the voltage pulses have no voltage drop due to the electrical discharge and the voltage is applied between the workpiece and the wire electrode, wherein the instability calculation unit is configured to calculate the degree of instability, by using a ratio of total time of the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time, to total time obtained by summing the discharge delay time of the normal discharge pulses, the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time.