The present invention provides a linker preferably used when constructing an mRNA/cDNA-puromycin-protein conjugate used in an in vitro virus method, and an mRNA/cDNA-puromycin-protein conjugate constructed using that linker. More specifically, the present invention provides a linker for ligating mRNA and puromycin or a puromycin-like compound to construct an mRNA/cDNA-puromycin-protein conjugate, the linker comprising a single-stranded RNA as a main backbone, and having, in this main backbone, a solid phase binding site for binding an mRNA-puromycin-protein conjugate to a solid phase site, and a pair of cleavage sites provided at locations surrounding the solid phase binding site; an mRNA-puromycin-protein conjugate constructed using this linker; an mRNA bead or an mRNA chip comprising this conjugate; a protein chip produced from this mRNA chip; and a diagnostic kit using the mRNA bead or the mRNA chip.