We studied field-induced conductance change of thin organic films using the trench-type electrodes, where the gate field is applied from the sidewall of the trench. We fabricated the electrodes, and observed field effects of a conductive self-assembled film and oligothiophene thin films. The electric field generated by the electrode structure was calculated by a two-dimensional finite element method. The gate field strength becomes weaker as the film thickness is reduced, while the fields generated by the source-drain potential increase. This result denies the 'normally-off' characteristics since a small source-drain voltage between the nano-gap could cause the conductance change corresponding to the large gate voltage. Although three-dimensional structures or interface roughness could enhance the effective gate fields, we speculated that the self-assembled film showing the field effects was not a monolayer, but most probably multilayers or mounds of aggregated molecules of more than 10 nm thickness.