Provided is a method of manufacturing a circuit device in which a circuit element is resin-sealed with sealing resins formed integrally with each other. In the present invention, a resin sheet and a circuit board are housed in a cavity of a mold, and thereafter a first sealing resin formed of a tablet in melted form is injected into the cavity. At the time of injecting the first sealing resin, a second sealing resin formed of the resin sheet in melted form is not yet cured and is maintained in liquid form. Accordingly, the injected first sealing resin and the second sealing resin are mixed at the boundary therebetween, preventing the generation of a gap in the boundary portion and therefore preventing the deterioration of the moisture resistance and withstand voltage at the boundary portion.