A urea water consumption diagnostic device for a urea selective catalytic reduction system configured to suck urea water from a urea water tank with a supply pump and spray the urea water from a dosing valve disposed upstream of a selective catalytic reduction device via a pressurized urea water feed line, the device comprising: a urea water sensor configured to detect a level of the urea water in the urea water tank; consumption volume calculating means configured to calculate a cumulative consumption volume from a detection value entered from the urea water sensor, and calculate a supplemented volume from a change in the level of the urea water sensor upon supplementation of the urea water, thereby correcting the cumulative consumption volume; and abnormality diagnosing means configured to, when a cumulative instructed spray volume of the urea water sprayed from the dosing valve becomes equal to or greater than a determination volume, compare the cumulative consumption volume calculated by the consumption volume calculating means to the cumulative instructed spray volume to detect defects and malfunctioning such as clogging of the dosing valve or the pressurized urea water feed line, and unintentionally continued spraying due to the dosing valve being stuck in the open condition, characterised by the urea water sensor being configured to detect a level of urea water in the urea tank in a stepwise manner; the consumption volume calculating means being configured to: store level values of the urea water sensor when a key switch is turned off and on; calculate a changed level value (S1 +n -S1) which is a difference between a level value (S1 +n) stored when the key switch is turned on and a level value stored when the key switch is turned off and before the key switch is turned on; and when the changed level value (S1 +n -S1) is greater than a predetermined value, determine that the urea water is supplemented to the urea water tank, subtract one level (R REF) from the changed level value, and store a resulting level value as the supplemented volume. a urea water sensor configured to detect a level of the urea water in the urea water tank; consumption volume calculating means configured to calculate a cumulative consumption volume from a detection value entered from the urea water sensor, and calculate a supplemented volume from a change in the level of the urea water sensor upon supplementation of the urea water, thereby correcting the cumulative consumption volume; and abnormality diagnosing means configured to, when a cumulative instructed spray volume of the urea water sprayed from the dosing valve becomes equal to or greater than a determination volume, compare the cumulative consumption volume calculated by the consumption volume calculating means to the cumulative instructed spray volume to detect defects and malfunctioning such as clogging of the dosing valve or the pressurized urea water feed line, and unintentionally continued spraying due to the dosing valve being stuck in the open condition, characterised by the urea water sensor being configured to detect a level of urea water in the urea tank in a stepwise manner; the consumption volume calculating means being configured to: store level values of the urea water sensor when a key switch is turned off and on; calculate a changed level value (S1 +n -S1) which is a difference between a level value (S1 +n) stored when the key switch is turned on and a level value stored when the key switch is turned off and before the key switch is turned on; and when the changed level value (S1 +n -S1) is greater than a predetermined value, determine that the urea water is supplemented to the urea water tank, subtract one level (R REF) from the changed level value, and store a resulting level value as the supplemented volume. store level values of the urea water sensor when a key switch is turned off and on; calculate a changed level value (S1 +n -S1) which is a difference between a level value (S1 +n) stored when the key switch is turned on and a level value stored when the key switch is turned off and before the key switch is turned on; and when the changed level value (S1 +n -S1) is greater than a predetermined value, determine that the urea water is supplemented to the urea water tank, subtract one level (R REF) from the changed level value, and store a resulting level value as the supplemented volume.