A radio frequency multilayer substrate which has a connection portion connecting a strip line and a microstrip line, of which connection portion improves VSWR, is provided. The high frequency multilayer substrate has a through-hole which electrically connects a central conductor of the strip line and a central conductor of the microstrip line. The high frequency multilayer substrate also has an insulating hole which does not have a conductor inside. The through-hole is connected with the insulating hole. A length of the conductor layer of the through-hole from the central conductor to an insulating hole is smaller than one half the distance from the central conductor to a 2nd ground conductor. The insulating hole can be formed by cutting the through-hole.