A lane detection and lane departure determining device and method are provided. The lane detection device includes a first Region of Interest (ROI) setting unit configured to set a rectangular ROI from an input image which is a road image, a second ROI setting unit configured to set a Λ-ROI having distorted trapezoidal shape from the rectangular ROI, and a lane detection unit configured to detect the lane including a left and right lane marking in the Λ-ROI. The second ROI setting unit is configured to calculate a merged first line segment through a merging process, obtains a left and right second line segment by scanning brightness, and determines the left and right second line segment as a temporary left and right lane marking when a preset condition is satisfied. The Λ-ROI is determined by changing a slope of the temporary left and right lane marking in a predetermined range.