Liquid reducing agent judging apparatus

[Subject] The sensor which outputs the signal relevant to the concentration of a liquid reducing agent from the heat transfer characteristic for two points of having estranged is used, and distinction of whether a liquid reducing agent is empty, different-species solution, or a regular thing is always enabled. [Solution means] If the measurement trigger outputted for every predetermined time after engine start is detected, while driving a sensor and measuring the concentration of urea solution, the concentration calculates the number of times of an empty judging or the number of times of a different-species judging as for which below the minimum threshold value that performs more than the upper limit threshold that performs an empty judging continuously, or a different-species judging became. And if the number of times of an empty judging or the number of times of a different-species judging becomes beyond a predetermined value, it will judge with urea solution being empty or different-species solution (S42*S45). Moreover, if the number of times of an empty judging or the number of times of a different-species judging is less than a predetermined value, it will judge with urea solution being regular (S46). [Selection figure] Fig. 8