In a flat pin to be used in a circuit board made of resin with pins comprising a rod portion having a diameter of not greater than 0.35 mm and a concentric tabular large diameter portion having a larger diameter than that of the rod portion formed on one end of the rod portion, the ratio (W/S) of the diameter of the large diameter portion to the rod portion is from not smaller than 2.16 to not greater than 2.67 and the ratio (T/S) of the thickness of the large diameter portion to the diameter of the rod portion is from not smaller than 0.40 to not greater than 0.67 supposing that the diameter of the rod portion and the large diameter portion of the flat pin are S and W, respectively, and the thickness of the large diameter portion is T.