Automated driving systems can perform dynamic driving tasks without drivers' intervention, thus leading to increasing driver engagement in non-driving-related tasks (NDRT). However, drivers' engagement in NDRT may result in a decrease in situation awareness and driving performance. There is a need to study the driver's level of monitoring the environment during NDRT. This study divided the resource demands of NDRT into cognitive, visual, and physical and examined the difference in driver's visual attention allocation patterns in NDRT with different resource demands. Meanwhile, four NDRTs was examined (none, cognitive, cognitive-visual, and cognitive-visual-physical) with different engagement time (5 min, 15 min). Results show that the driver devoted high attention resources to the driving environment in the control group and cognitive tasks. While the engagement time significantly affects the driver's attention level to the driving environment in the control group. The driver's attention to the driving environment is significantly reduced in the cognitive-visual and the cognitive-visual-physical tasks. Furthermore, it is more difficult for drivers to disengage from NDRT to observe the driving environment in tasks that occupy physical resource demands.