Composite substrate, module, and method of producing composite substrate

Provided are a composite substrate, a module comprising the composite substrate, and a method of producing the composite substrate, that are capable of dispersing the stress applied to external connecting terminals by dropping, and that can sufficiently ensure the connection strength of a ceramic substrate and the external connecting terminals. The composite substrate according to the present invention comprises: the ceramic substrate (1), at least one side of which has circuit wiringfor mounting an electronic component (2) formed thereon; the plurality of external connecting terminals (3) that are formed on one side of the ceramic substrate (1); and a resin layer (5) that is formed on the one side of the ceramic substrate (1). The external connecting terminals (3) are of a shape such that the cross-sectional area becomes smaller the further away from the one surface of the ceramic substrate (1), and a portion or all of the terminal surface that is on the opposite side of the terminal surface connected with t