Pitching is one of the most important elements of baseball. Therefore, both professional and amateur players are interested in measuring their pitching performance, particularly their pitch speed. Conventional equipment such as radar speed guns can be used to measure pitch speed; however this approach is unpopular among amateur players because of its cost. Therefore, we proposed a vision-based speed-measuring method for baseball pitches and developed a smartphone application called iPhoneSG, which can measure the speed of pitched baseballs on a real baseball diamond in near-real-time using a standalone image-processing technique on the smartphone. Relative to conventional radar speed guns, which must be placed on the extended line of the ball trajectory, iPhoneSG widens the possible area of data acquisition. Considering the popularity of smartphones, iPhoneSG provides players with a convenient means of measuring pitch speeds. We confirmed the basic viability of the proposed method as implemented in our iPhoneSG application.