Disclosed is an immunostimulator containing virus-like particles, in which the virus-like particles contain an outer coat protein containing an amino acid sequence selected from the group consisting of SEQ ID No. 1, SEQ ID No. 26, SEQ ID No. 33, SEQ ID No. 35, SEQ ID No. 37, SEQ ID No. 39, SEQ ID No. 41, SEQ ID No. 43 and SEQ ID No. 45 the outer coat protein constitutes an outer coat of the virus-like particles and the virus-like particles do not substantially contain a genome DNA of SV40.
