For soldering a terminal (14) to a land (13) formed on an FPC (12) made of a material having a softening point below 230°C, such as PET, by using a flow soldering or reflow soldering technique, an opening (17) larger than the land (13) is formed in a thermal insulation holder (11) including a phenolic paper base copper clad laminate board or the like so that the land (13) and the terminal (14) are within sight, and the thermal insulation holder (11) is held in contact with a surface of the FPC (12) to which solder is applied so that the land (13) and the terminal (14) are located within the opening (17) to perform the soldering using the flow soldering or reflow soldering technique, whereby molten solder readily reaches the land (13) through the opening (17) which need not be worked into a conventional trapezoidal configuration in vertical section, thereby preventing poor soldering.