"Speed lines" is the abstract lines used in comic to make objects look like they're moving. The number and the length of speed lines are manipulated by comic artists to match the speed they want to express, but few studies have tested their effect on perception of motion of the viewer. To investigate the matter, we conducted an experiment employing prediction-motion task. 19 participants were shown an apparent motion of a ball moving in uniform linear motion, and were told to estimate the speed of the ball. Each ball was presented with one of six patterns of speed lines, 3(number) x2(length). As result, we found that both the number and the length of speed lines have significant effect on subjective speed of the ball. This indicates that the number and the length of speed lines indeed affects viewer's perception of motion, thus plays significant role in comics.