The helical antenna comprises a base body and a helically-configured conductor on a top surface of the base body, wherein base body's thickness a is given as 0.3≦a≦3 (mm); length b is given as 5≦b≦20 (mm); and relative dielectric constant &egr;r is given as 3≦&egr;r≦30, and conductor's winding number x is given as 3≦x≦16 (turns), and its resonant frequency f and conductor width w satisfy formulae (1): f=Ax+By+C (MHz) and (2): w=Dx+E (mm), where y represents base body width and A through E represent constants determined according to the base body's thickness a, length b, and relative dielectric constant &egr;r.