A seat stem positioning structure for a chair for drummer, including an internally threaded seat stem connector shaped like a stepped tube and fastened to a tubular central shaft and equipped with pivoted legs for supporting on the ground, a threaded seat stem having a screw section threaded into the seat stem connector to hold a seat for sitting, and a clamping device fastened to the seat stem connector and the threaded seat stem by fastening means to firmly secure them together, the clamping device including two hinged clamping plates having a respective connecting plate at the respective free end fastened to each other by a screw and a wing nut, and a respective downward locating rod forced into engagement with a respective locating notch in a step at the seat stem connector.