Staphylococcus epidermidis (S. epidermidis) often cause sepsis and related diseases by transfusion of contaminated platelet concentrates (PCs). The proliferation process of this bacterium in PCs has been unclear, thus, bio-imaging system was applied for analyzing the dynamics of S. epidermidis in PCs. S. epidermidis were spiked into PCs or Luria Bertani (LB) broth. These samples were collected at each sampling time during incubation (up to 7 days), and colony-forming-units were counted. Bacterial number and their size distribution in each sample were also determined with a new bio-imaging system. The morphological characters of S. epidermidis growing in the samples were observed precisely by scanning electron microscopy (SEM). The numbers of S. epidermidis were stable for 48 hr after the spiking as lag-phase, while the bio-imaging analysis also showed that aggregates proliferated during “lag-phase.” The aggregates were also observed in LB media, however, their sizes were much smaller than those in PCs. SEM suggested that the aggregates were micro-colonies (MCs) of staphylococcal cells and cores of the MCs are composed with platelets (PLTs). Out results suggested that S. epidermidis formed floating MCs in PCs during “lag-phase.” Therefore, the term of lag phase of S. epidermidis in PCs should be called as “pseudo-lag phase.” The initial processes of forming MCs in PCs are thought to be an interaction between bacterial cells and PLTs. Floating MCs would be the source of biofilms on the inside of PC storage bags. New information obtained in this study would be useful for understanding the dynamics of growing bacteria in PCs.