Effects of beam convergence on fusion reaction rate in the cylindrical IECF device are investigated using 2-D simulation code by changing electrode shape. Simple simulation well reproduces experimentally obtained dependences of neutron production rate vs. discharge voltage and current, and it is found that almost all reactions occur by beam-background gases collisions, not beam-beam collisions. Results show that fusion reaction rate increases significantly with increase of beam convergence, therefore, increase of ion densities in the center, even in the devices where beam-background gas collision is dominate.