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We present an iteration method to derive exact rotation curves (RC) of spiralgalaxies from observed position-velocity diagrams (PVD), which comprises thefollowing procedure. An initial rotation curve, RC0, is adopted from anobserved PV diagram (PV0), obtained by any simple method such as thepeak-intensity method. Using this rotation curve and an observed radialdistribution of intensity (emissivity), we construct a simulated PV diagram(PV1). The difference between a rotation curve obtained from this PV1 and theoriginal RC (e.g., difference between peak-intensity velocities) is used tocorrect the initial RC to obtain a corrected rotation curve, RC1. This RC1 isused to calculated another PVD (PV2) using the observed intensity distribution,and to obtain the second iterated RC (RC2). This iteration is repeated untilPV$i$ converges to PV0, so that the differences between PV$i$ and PV0 becomesminimum. Finally RC$i$ is adopted as the most reliable rotation curve. We applythis method to some observed PVDs of nearby galaxies, and show that theiteration successfully converges to give reliable rotation curves. We show thatthe method is powerful to detect central massive objects.Comment: To appear in ApJ.Letters, 5 pages Latex with 4 figure

Iteration Method to Derive Exact Rotation Curves from Position-Velocity Diagrams of Spiral Galaxies