Understanding the relationship between the microscopic structures and the carrier transport properties in organic electronic materials is of fundamental interest to exploit the potential of the materials and thus to realize highly efficient organic devices. In this study, carrier ejection processes in polycrystalline organic thin film transistors are investigated by time-resolved Kelvin probe force microscopy (tr-KPFM) that is capable of visualizing time evolution of the surface potential triggered by the application of a voltage pulse with a temporal resolution of 3 ms. The obtained tr-KPFM images show that the spatial distribution of the diffusing charge carriers is mostly non-homogeneous, reflecting the complex resistivity distribution in the channel region formed by the crystalline grains. The trapped charges in some grains diffuse to the electrodes on a time scale of several tens of milliseconds due to the low contact resistance of the grain boundaries, while most of the trapped charges diffuse on a time scale of hundreds of milliseconds.