This thesis is aimed at studying and analyzing the criteria in determining an implied agency as stipulated in the Civil and Commercial Code. It also analyzes related problems by comparing Thai and foreign laws regarding an implied agency. From the studies, it was found that an implied agency is different from an agency by estoppel both in description and legal consequence. For an implied agency, the principal shows his consent and obligation for any action committed by an implied agency even though he does not appoint the implied agent or expressly give consent to it. But Section 820 will be applied with regard to the obligation and bond between the third person and the principal because as the fact the principal is willing to accept the agent with implied acceptance. เท the case of an agency by estoppel, the principal does not express consent or approve the action of agency by estoppel. The principal merely ignores to disclose the facts to the innocent third person that an agency estoppel performs an action on his behalf. The law, thus, does not allow the principal to argue that the person is not his agent. With regard to an agency by estoppel, a principal will consequently be liable under Section 821. Section 820 is not applied to this case because it must be applied under the operation of law. Besides, in connection with the authority of an implied agency, there is no clear-cut mention of an implied authority in Thai legislation. However, the general principal of law shall be applied in this case. Concerning the relationship, rights, duties and obligations between a principal, an agency and a third person, the provision of law of an agency can be applied as if it does not against the theory and principle of law on agency, such as Section 810, 812 or 820 etc. As an implied agency and an agency by estoppel are different in nature and in order to comply with the spirit of the provision of an implied agency. I would like to suggest that it is necessary to interpret and apply these two kinds of agency separately and correctly. An agency by estoppel must not be regarded as a kind of implied agency. Moreover, the lawyers must realize the legal status of an implied agency clearly, including rights, duties and obligations of all parties concerned.
