This paper proposes an accelerator-based neutron beam to evaluate thermal-neutron-induced soft-error rate (tnSER) in semiconductor devices. The thermal-neutron flux is sufficient for the tnSER evaluation in the beam, and the moderator for the beam is compact, and we can perform the tnSER test at many facilities. We have evaluated tnSER in an SRAM with the proposed beam and a nuclear reactor, and the tnSER is in good agreement between the tests with the proposed neutron beam and the nuclear reactor.