The principle of good faith means that people living together in society must act in good faith and with sincerity so as not to betray the other party's trust. Because this content is highly abstract and ethical, defining it concretely and clearly is difficult. The Customs Act states the principle of good faith by providing that ‘taxpayers shall perform their duties in good faith and with sincerity, and the same applies to customs officials in performing their duties.’

There is some debate over whether the principle of good faith applies only to the tax authorities or must be observed by taxpayers as well as the authorities, but the Customs Act requires it of both. However, considering the various adverse dispositions a taxpayer faces for acting against their own past words or conduct toward the tax authorities, the authorities' superior position, and the fact that the burden of proving that a tax disposition is lawful lies in principle with the tax authorities, the principle of good faith should be applied to taxpayers only in very limited cases and must not be interpreted broadly.

The main body examined the three basic principles of tax law: no taxation without law, tax equality and good faith. In carrying out taxation, these three basic principles need to be realized. The characteristics that tax legislation must observe include public character, administrative primacy, self-sufficiency and equality, technical and written character, and mandatory force.

To add: the constitutional basis of the principle of no taxation without law is Article 38 (duty to pay taxes) and Article 59 (types and rates of taxes are set by law), and tax equality comes from the equality principle in Article 11. The principle of good faith is written not only in Article 6 of the Customs Act but in the same words in Article 15 of the Framework Act on National Taxes, and Article 18 of that act requires tax laws to be interpreted so as not to unduly infringe taxpayers' property rights and bans retroactive taxation. The Supreme Court holds that applying the good faith principle against the tax authorities requires all of the following: an official statement of position, the taxpayer's reliance on it without fault, action taken on that reliance, and a disposition contrary to the stated position that causes the taxpayer harm.