PURPOSE: To diagose precisely various cancers by determination of sugar branch which is produced by cancer cells and exists in body fluids of cancer patients by use of a specific reaction with lectin through a competition or a sandwith method.

CONSTITUTION: A sugar branch related to cancer (expressed as TAG hereunder) or a TAG like substance [e.g. galactose (β1→3 or β1→4)-N-acetylglu-cosamine] which is produced by cancer cells in body fluids of cancer patients is reacted with an insoluble carrier, then an immobilized TAG or TAG like substance is prepared subsequently, and a definite amount of a body liquid of a cancer patient contg. TAG to be measured and an immoblized TAG (TAG like substance) are reacted competitively with a definite amount of a labeled lectin. Then a complex of the immobilized TAG (TAG like substance) and the labeled lectin and a free lectin are separated, TAG in a body fluid is determined by measuring of either of a labeled agent activities.