It is believed that satellites of giant planets form in circumplanetary disks (CPDs). Many of the previous contributions assumed that their formation process proceeds similarly to rocky planet formation via accretion of the satellite seeds called satellitesimals. However, the satellitesimal formation itself poses a nontrivial problem, as the dust evolution in CPD is heavily impacted by fast radial drift and thus dust growth to satellitesimals is hindered. To address this problem, we connected state-of-the art hydrodynamical simulations of a CPD around a Jupiter-mass planet with dust growth, and a drift model in a post-processing step. We found that there is an efficient pathway to satellitesimal formation if there is a dust trap forming within the disk. Thanks to natural existence of an outward gas-flow region in the hydrodynamical simulation, a significant dust trap arises at the radial distance of 85 R J from the planet, where the dust-to-gas ratio becomes high enough to trigger streaming instability. The streaming instability leads to efficient formation of the satellite seeds. Because of the constant infall of material from the circumstellar disk and the very short timescale of dust evolution, the CPD acts as a satellitesimal factory, constantly processing the infalling dust to pebbles that gather in the dust trap and undergo the streaming instability.