A urea water suitability determination device comprising: a concentration detecting section, which detects a concentration of urea water and outputs the detected value; a determining section, which determines whether the urea water is suitable by using the detected value output from the concentration detecting section; and a temperature detecting section (29, 35, 45), which detects temperatures (Tex, Tu, Tw) of a plurality of detection targets at different locations in a vehicle, wherein the detection targets have temperatures that are different from one another during operation of an engine, the determining section is adapted to: calculate a temperature difference between a temperature of a particular detection target and a temperature of another detection target among the detection targets; and when a determination start condition is satisfied, start determining whether the urea water is suitable, the determination start condition includes that the temperature difference is within a reference range, the reference range is defined as a range of temperature differences in which it is determined that the urea water is in a quiescent state, which is suitable for the determination, the detection targets include the urea water, the concentration detecting section is adapted to detect a concentration of the urea water based on a temperature (Tu) of the urea water and a propagation velocity (Vus) of an ultrasonic wave in the urea water, the determining section is adapted to: determine suitability of the urea water at a first determining opportunity, where the urea water is at a first temperature, and determine suitability of the urea water at a second determining opportunity, where the urea water is at a second temperature, which is different from the first temperature, the urea water suitability determination device further comprises a heating section for heating the urea water, the determining section is adapted to: when the urea water is determined as suitable at the first determining opportunity, heat the urea water by the heating section, determine whether a changing amount (ΔTu) of the urea water temperature from the first temperature exceeds a preset amount (ΔTut), and when the changing amount (ΔTu) exceeds the preset amount (ΔTut), determines suitability of the urea water at the second determining opportunity. a concentration detecting section, which detects a concentration of urea water and outputs the detected value; a determining section, which determines whether the urea water is suitable by using the detected value output from the concentration detecting section; and a temperature detecting section (29, 35, 45), which detects temperatures (Tex, Tu, Tw) of a plurality of detection targets at different locations in a vehicle, wherein the detection targets have temperatures that are different from one another during operation of an engine, the determining section is adapted to: calculate a temperature difference between a temperature of a particular detection target and a temperature of another detection target among the detection targets; and when a determination start condition is satisfied, start determining whether the urea water is suitable, calculate a temperature difference between a temperature of a particular detection target and a temperature of another detection target among the detection targets; and when a determination start condition is satisfied, start determining whether the urea water is suitable, the determination start condition includes that the temperature difference is within a reference range, the reference range is defined as a range of temperature differences in which it is determined that the urea water is in a quiescent state, which is suitable for the determination, the detection targets include the urea water, the concentration detecting section is adapted to detect a concentration of the urea water based on a temperature (Tu) of the urea water and a propagation velocity (Vus) of an ultrasonic wave in the urea water, the determining section is adapted to: determine suitability of the urea water at a first determining opportunity, where the urea water is at a first temperature, and determine suitability of the urea water at a second determining opportunity, where the urea water is at a second temperature, which is different from the first temperature, determine suitability of the urea water at a first determining opportunity, where the urea water is at a first temperature, and determine suitability of the urea water at a second determining opportunity, where the urea water is at a second temperature, which is different from the first temperature, the urea water suitability determination device further comprises a heating section for heating the urea water, the determining section is adapted to: when the urea water is determined as suitable at the first determining opportunity, heat the urea water by the heating section, determine whether a changing amount (ΔTu) of the urea water temperature from the first temperature exceeds a preset amount (ΔTut), and when the changing amount (ΔTu) exceeds the preset amount (ΔTut), determines suitability of the urea water at the second determining opportunity. when the urea water is determined as suitable at the first determining opportunity, heat the urea water by the heating section, determine whether a changing amount (ΔTu) of the urea water temperature from the first temperature exceeds a preset amount (ΔTut), and when the changing amount (ΔTu) exceeds the preset amount (ΔTut), determines suitability of the urea water at the second determining opportunity.