A magnetic field measurement under a warm condition for a prototype of superconducting combined function magnet (SCFM) in a J-PARC was performed. In this measurement, the magnet was aligned on an alignment-line defined by a laser beam by referring alignment markers on the magnet shell and laser beam position markers on the alignment-line. Each process of alignment and measurement was performed within plusmn0.05 mm accuracy. To identify the precise magnetic field distribution in the SCFM, we developed a position-monitoring system of a magnetic sensor (rotating coil), which consisted of a laser also used as the alignment-line and a position-sensitive detector mounted on the rotating coil. By using this position monitor, we identified displacements of the rotating coil to plusmn0.06 mm accuracy. Finally, we succeeded to identify the measured positions of the magnetic field in the prototype SCFM within the required tolerance of plusmn0.3 mm