The photocount distribution from a quasi-thermal light source, a moving ground glass disk (surface roughness; 9 ${mu}{ extrm}{m}$) illuminated by a well-stabilized He-Ne laser, is measured by a photon counting system, and analyzed with theoretical calculations. The distribution approaches the Poisson distribution for the long coherence time ${ au}_c$ compared to the measuring time T. The coherence time ${ au}_c$ of the quasi-thermal source can be changed by controlling the velocity v of the motor driving the glass disk. By the comparison of experimental results and theory for the condition of T/ ${ au}_c$ >>1, the coherence time ${ au}_c$ of the quasi-thermal source is turned out to be in the range of 31.43 $mutextrm{s}$~2.48 $mutextrm{s}$ according to the circumferential velocity of the disk, and compared with the simple calculation of $sigma$/v.