It is not rare case that a floated metal plate exists nearby high speed circuit traces. Heatsink placed on a IC chip nearby circuit traces, metal enclosure or circuit traces in a compact designed product may be a good example. It may be also seen such structure for a shield box and circuit traces confined. It is generally known that such metal plate as placing nearby circuit trace may change circuit trace parameters and then resonance frequency associated with the circuit trace. In this study, we clarified resonance frequency variation with comprehensive observation of input impedance of microstrip line that is an essential model of circuit traces on a printed circuit board. Since such structure is created in various cases in product designs, we believe that the results shown in this study may be useful for EMC design as well as signal integrity. For computation, method of moment was used.