To provide a photoresist base material and its composition which can be used for ultramicro-fabrication using extreme UV rays or the like.

The photoresist base material comprises an organic compound exhibiting reactivity to extreme UV light and expressed by general formula (1). In formula (1): A represents a center structure comprising a 1 to 50C aliphatic group, a 6 to 50C aromatic group, an organic group having both of them, or an organic group having a cyclic structure with repetition of those groups; each of B to D represents a group reactive to extreme UV light, a group reactive to the effect of a chromophore active to extreme UV light, or a 1 to 50C aliphatic group or a 6 to 50C aromatic group including the above reactive groups, or an organic group having those groups, or a substituent having a branched structure; each of X to Z represents a single bond or an ether bond; and each of l to n represents an integer from 0 to 5 satisfying l+m+n≥1. The groups A to D may contain a substituent having a heteroatom.