A liquid quantity detecting device for a vehicle, the vehicle including an internal combustion engine, an exhaust gas control device configured to add an addition liquid for purifying exhaust gas to an exhaust passage of the internal combustion engine, a tank that stores the addition liquid, and a level sensor that detects a liquid surface level of the addition liquid stored in the tank, the liquid quantity detecting device comprising: an update unit configured to i) update a residual quantity of the addition liquid in the tank, ii) execute a first update process which is a process of subtracting a total quantity of the addition liquid from a last-updated residual quantity of the addition liquid, the total quantity being a quantity of the addition liquid added to the exhaust passage during a period from last update of the residual quantity of the addition liquid to current update of the residual quantity of the addition liquid, and iii) set a value obtained through subtraction as an updated residual quantity of the addition liquid; and a notification unit configured to give a notification on the residual quantity of the addition liquid when the residual quantity of the addition liquid updated by the update unit is equal to or smaller than a predetermined value; and characterized in that the update unit is configured to execute the first update process when a first condition including a condition that the vehicle is stationary is fulfilled, and in that the update unit is configured to execute a second update process when a second condition is fulfilled, the second update process being a process of setting a residual quantity of the addition liquid based on a sensor value detected by the level sensor as the updated residual quantity of the addition liquid, and the second condition including a condition that i) the vehicle travels for more than a predetermined time and ii) a fluctuation width of the sensor value of the level sensor remains equal to or smaller than a predetermined value for more than the predetermined time. an update unit configured to i) update a residual quantity of the addition liquid in the tank, ii) execute a first update process which is a process of subtracting a total quantity of the addition liquid from a last-updated residual quantity of the addition liquid, the total quantity being a quantity of the addition liquid added to the exhaust passage during a period from last update of the residual quantity of the addition liquid to current update of the residual quantity of the addition liquid, and iii) set a value obtained through subtraction as an updated residual quantity of the addition liquid; and i) update a residual quantity of the addition liquid in the tank, ii) execute a first update process which is a process of subtracting a total quantity of the addition liquid from a last-updated residual quantity of the addition liquid, the total quantity being a quantity of the addition liquid added to the exhaust passage during a period from last update of the residual quantity of the addition liquid to current update of the residual quantity of the addition liquid, and iii) set a value obtained through subtraction as an updated residual quantity of the addition liquid; and a notification unit configured to give a notification on the residual quantity of the addition liquid when the residual quantity of the addition liquid updated by the update unit is equal to or smaller than a predetermined value; and characterized in that the update unit is configured to execute the first update process when a first condition including a condition that the vehicle is stationary is fulfilled, and in that the update unit is configured to execute a second update process when a second condition is fulfilled, the second update process being a process of setting a residual quantity of the addition liquid based on a sensor value detected by the level sensor as the updated residual quantity of the addition liquid, and the second condition including a condition that i) the vehicle travels for more than a predetermined time and ii) a fluctuation width of the sensor value of the level sensor remains equal to or smaller than a predetermined value for more than the predetermined time.