<p>An engine (1) is provided with an SCR catalyst (41) for purifying NOx and a urea injection valve (230) for adding urea water to the SCR catalyst (41). A control device (80) causes urea water, in an amount corresponding to the NOx emission volume, to be injected from the urea injection valve (230) by way of controlling the opening and closing cycle of the urea injection valve (230) and the open valve period during the opening and closing cycle. In addition, the control device (80) lengthens the open valve period for the urea injection valve (230) when the temperature of the urea injection valve (230) is low, and lengthens the opening and closing cycle of the urea injection valve (230) when the open valve period is long.</p>
