The present invention relates to a position detection device for a mobile robot, including a light-emitting element for emitting light, a light reception element on which light transferred from a bottom, of the light emitted from the light-emitting element and then reflected from an obstacle, is focused, and a signal processing unit for calculating a distance between a position where the light incident on the light reception element is focused and the obstacle. A position detection method using a mobile robot may include operating a robot cleaner, receiving light transferred from a bottom, of light emitted from the robot cleaner and then reflected from an obstacle, and detecting a distance between the robot cleaner and the obstacle from which the light is reflected, determining whether the distance between the robot cleaner and the obstacle increases, and determining whether the robot cleaner has approached the obstacle.