From a long-term perspective, fixed costs and unit variable costs can be said to have an inverse relationship. This paper resolved the problem of divisional misreporting that can arise when the truthful reporting assumption inherent in traditional common cost allocation methods is relaxed in cases where fixed costs and unit variable costs have such an inverse relationship, by employing Groves' incentive-compatible mechanism. The common cost allocation method proposed in this paper eliminates the incentive for divisional misreporting by ensuring that the common costs allocated to a division are not affected by that division's own reports. Furthermore, when a division does misreport, the corporate losses resulting from the misreporting are borne by the misreporting division, thereby inducing truthful reporting by divisions. By using the common cost allocation method proposed in this paper, corporate headquarters can make common resource investment decisions that maximize firm-wide profits based on divisions' truthful reports, and can conduct relatively fair performance evaluations of divisions based on their actual performance.