A conditional processing method of an apparatus comprising a memory, comprising the steps of: receiving a data stream including a plurality of elements, wherein the plurality of elements are structured based on a declarative description language; decoding the received data stream; loading the decoded data stream in a memory; generating a parse tree using the elements constituting the decoded data stream, wherein generating a parse tree comprises: determining whether a parsing switch element for performing conditional processing in a data parsing and loading process is detected while generating the parse tree; if a parsing switch element is detected, loading attributes of one or more child elements of the detected parsing switch element in the memory, and evaluating the one or more child elements in series with their attributes loaded in the memory, wherein the loading step comprises: loading, if a mode attribute of the detected parsing switch element is decremental, data of a first child element of the parsing switch element that satisfies the one or more attributes of the detected parsing switch element into the memory, wherein the one or more attributes define a required memory amount as a criterion for the conditional processing, among child elements of the parsing switch element that are listed in descending order with respect to memory requirements, and unloading or deleting a child element of the parsing switch element that does not satisfy the attributes of the detected parsing switch element from the memory. receiving a data stream including a plurality of elements, wherein the plurality of elements are structured based on a declarative description language; decoding the received data stream; loading the decoded data stream in a memory; generating a parse tree using the elements constituting the decoded data stream, wherein generating a parse tree comprises: determining whether a parsing switch element for performing conditional processing in a data parsing and loading process is detected while generating the parse tree; if a parsing switch element is detected, loading attributes of one or more child elements of the detected parsing switch element in the memory, and evaluating the one or more child elements in series with their attributes loaded in the memory, determining whether a parsing switch element for performing conditional processing in a data parsing and loading process is detected while generating the parse tree; if a parsing switch element is detected, loading attributes of one or more child elements of the detected parsing switch element in the memory, and evaluating the one or more child elements in series with their attributes loaded in the memory, wherein the loading step comprises: loading, if a mode attribute of the detected parsing switch element is decremental, data of a first child element of the parsing switch element that satisfies the one or more attributes of the detected parsing switch element into the memory, wherein the one or more attributes define a required memory amount as a criterion for the conditional processing, among child elements of the parsing switch element that are listed in descending order with respect to memory requirements, and unloading or deleting a child element of the parsing switch element that does not satisfy the attributes of the detected parsing switch element from the memory. loading, if a mode attribute of the detected parsing switch element is decremental, data of a first child element of the parsing switch element that satisfies the one or more attributes of the detected parsing switch element into the memory, wherein the one or more attributes define a required memory amount as a criterion for the conditional processing, among child elements of the parsing switch element that are listed in descending order with respect to memory requirements, and unloading or deleting a child element of the parsing switch element that does not satisfy the attributes of the detected parsing switch element from the memory.