This paper proposes a virtual network function (VNF) placement model for service chaining that minimizes the cost of using computation resources when no failure occurs while guaranteeing recovery against any single facility node failure within the recovery time objective (RTO) defined for each service. The proposed model adaptively allocates computation resources to each service under its RTO constraint. The proposed model introduces two sharing methods of computation resources among multiple service chains. The first method allows sharing a virtual machine (VM) where a VNF is scheduled to run after a failure, which contributes to suppressing the number of VMs reserved in preparation for a failure. The second method allows sharing computation capability used for VMs, which prevents unnecessary VNF scale-up that requires additional computation resources. A simulation study verifies that the proposed model reduces the cost of using computation resources compared to comparative models.