1. A fusion protein that can be expressed as a soluble protein, comprising: (1) a first amino acid sequence comprising the amino acid sequence of a polypeptide that is represented by the formula (Z) n , wherein n represents an integer from 1 to 5 and Z represents polypeptide selected from the group of: (a) a polypeptide comprising the amino acid sequence of SEQ ID NO: 1, (b) a polypeptide comprising an amino acid sequence that has at least 95% identity with the amino acid sequence of SEQ ID NO: 1, and (c) a polypeptide comprising an amino acid sequence that has at least 98% identity with the amino acid sequence of SEQ ID NO: 1, and that has the capacity when expressed as a fusion protein with a target protein to render said fusion protein expressible as a soluble protein; and (2) a second amino acid sequence comprising the amino acid sequence of a target protein, wherein the fusion protein further comprises, on the amino terminal side of the first amino acid sequence, an amino acid sequence that facilitates purification, and; wherein the expression of the fusion protein comprising the amino acid sequence that facilitates purification, an amino acid sequence of the polypeptide that is represented by the formula (Z) n and an amino acid sequence of the target protein is induced in Escherichia coli , and the fusion protein accumulates within the Escherichia coli as a soluble protein so that the fusion protein collected from within the Escherichia coli has higher solubility rate than non-(Z)n fusion protein; and wherein the amino acid sequence that facilitates the purification is a histidine tag sequence; and wherein the target protein is at least one selected from the group of apoaequorin, Gaussia luciferase, and Oplophorus luciferase.