We observed a conductive filament in CBRAM (conductive bridging RAM) during switching operations by in-situ TEM (transmission electron microscopy) method. A vertically stacked-type CBRAM is popular for TEM observation, however, we used a lateral-type CBRAM (Cu-WOX-Cu CBRAM) for clearer TEM observations. In a Set operation (resistive switching from the high resistance state (HRS) to the low resistance state (LRS)), a conductive filament grew from a cathode to an anode and increase of Cu in the cathode was observed. After the Set operation, a Reset operation (resistive switching from LRS to HRS) was observed in the opposite voltage polarity to the former Set but big structural changes were not seen. The growth direction of Cu conductive filament and increase of Cu in the cathode after the Set can be explained by a proposed switching model of CBRAM (conductive filament model).