There are provided an organic semiconductor device and a method of manufacturing the same, which make it possible to easily form a dense polymeric insulating film with high insulating properties as a gate insulating film, without using a vacuum apparatus, and to dispense with the step of patterning the gate insulating film. Gate electrodes 12 are formed on a glass substrate 11. Then, poly(1,4-bis(2-methylstyryl)benzene) (bis-MSB) is dissolved in benzonitrile containing 0.1 mol/l of tetrabutylammonium tetrafluoroborate, whereafter the glass substrate 11 having the gate electrodes 12 formed thereon is soaked in the solution to thereby form dense poly(bis-MSB) films by electrochemical polymerization.