Visual perception of motion depends on motion detection threshold, and motion with slower velocity than threshold is not perceived. Visual motion information is thought to pass through three stages: local motion processing, spatial contrast of motion, and integration of motion directions. However, it is not clear in which stage motion detection threshold is determined physiologically. The purpose of our study is to test whether motion detection threshold is affected by induced motion (Dunker, 1929), in which a central stimulus, even if it is stationary, appears to move in the opposite direction to the motion in the surround. Since this phenomenon is arguably related to the stage of spatial contrast of motion, induced motion should affect detection threshold if it is determined after the spatial contrast. Thus, we compared the detection thresholds for motions with and without integration with induced motion. We presented a central Gabor patch moving leftward or rightward together with a grating moving upward or downward within a surrounding annulus. The surround moved in one of nine speeds, stayed stationary, or was left blank. When the surround moved (e.g., upward), the central Gabor patch was induced to move perceptually in the opposite direction (e.g., downward), and if it was physically moving (e.g., rightward), the illusory direction and physical direction were integrated (e.g., appeared to move down-right). We determined the motion detection threshold of the Gabor patch for each condition by two-alternative forced-response task of directional judgment (left vs. right). The result showed that, at a certain range of surround motion speeds, the threshold for motion integrated with induced motion (perceptually moving diagonally) was lower than the threshold for motion not integrated with induced motion (i.e., conditions of stationary or blank surround). Our result indicates that motion detection threshold is physiologically determined at a stage after integration of motion direction.