At the solder ball mounting process on the CSP(chip size package), must have inspection of solder ball residue on the vacuum nozzle(inhaling tool). This study is how to detect solder ball residue during vacuum nozzle moving when using photo sensor. First, the characteristic of photo sensor output value in the case in which all inhaling entrance contain the solder ball and case in which it is not so was clarified on one or several inhaling area. Next, when the inhaling area is several, in order to eliminate the effect of the light leak on the output value of photo sensor, the differential coupled method of photo sensor was proposed, and the effectiveness was confirmed. In addition, arrangements of photo sensor and mobile speed of the inhaling tool, etc. were examined, and the development of the practicable detection method was tried. With this result, we discover following. (1) The solder ball recognition on CSP can be detect by photo sensor. (2) We confirmed effectiveness of the differential coupled method of photo sensor. (3) Mobile speed of inhaling tool dose not influence the output value, when differential coupled method of photo sensor was used. It is indicated that this detection method is practicable for this fact.