Abstract: A transformer core assembly in a transformer comprises a coil having a primary winding and a secondary winding which are successively wound about a hollow coil bobbin, in division, with respect to a longitudinal direction of the hollow coil bobbin, and a pair of E-type cores in which central core sections of respective three core sections extending in parallel relation to each other are inserted respectively from both ends of the coil bobbin into a hollow section therewithin. The core sections of one of the pair of E-type cores have lengths thereof which are formed shorter than lengths of the core sections of the other E-type core. When the central core sections are inserted respectively into the hollow section within the coil bobbin, so-called core gaps defined such that the core sections have respective forward ends thereof which are abutted against each other are located within an area corresponding to the primary winding of the coil bobbin.