Students majored in Computing are required to acquire programming Skills for fifty years. In introductory programming classes, after students solve ten of programming exercises, their ability to build correct programs is undeveloped. We introduced an online judge system which automatically judge students' program to be correct in order that students can recognize Whether their programs are correct. Some students acquire the ability to build correct programs by using the judge system. On the other hand, some students used the judge system by try and error manner so that they could acquire the expected ability. We conclude that ratio between submitted program and correct ones is important factor in order to check weather a student acquire the ability.