In obligation relations, a debtor bears main performance obligation and collateral obligation in contract. In addition to them, separate protection obligation is also recognized. Among other things, the collateral obligation and the protection obligation are of use, in the obligation structure theory under the law of obligation, to construct the legal theory as contract liability with respect to damages liability of a debtor which results from the damage by infringing on the life, body, or property of a creditor as well as on the performance interest of a creditor, that is, the recognition of protection obligation results in expansion of contract liability in its function. The theory of such protection obligation can be a valuable tool to present a solution to the opinion conflict in the academic world, i.e., to the deep-rooted conflict as to whether contract liability or tort liability.
From the perspective of comparative law, such a tendency can be found out, in spite of slightly different expressions, in the CISG that was drafted for unification of private law by the sheer acceptance of the contract liability theory in Anglo-American law, the PECL, the unified contract law of the Europe, and the recently revised German law of obligations. Thus, where a debtor in contract relation causes, in his performance, damage to the life, body, or property of a creditor, the debtor's liability should be considered as non-performance liability(contract liability) which violates protection obligation as collateral obligation of a debtor and this interpretation corresponds with the interpretation of Section 390 of the Civil Code. In order to put an end to the debates as to the matter of so-called recognition of protection obligation or individuality thereof, this author is of an opinion that it is desirable to codify it in the near future.