At first in this study the failure distribution of an interlocking fire door with a smoke detector was determined by analyzing the periodical inspection records. Next based on the failure distribuion the interval availability of the equipment, to which the periodical inspection and the corrective maintainance were executed, was evaluated. By considering the failure modes of elements of the equipment, an interlocking fire door with a smoke detector was divided into two units. The failure distribution of the unit 1, composed of the elements which are broken by sudden failures, is exponential with the failure rate λ=l/4, 341 [failures/day]. While, to the unit 2 mainly composed of electric elements, there may exist some uncertainty on determining the failure distribution whether the exponential distribution with the failure rate λ=1/13, 504 [failures/day] or the normal distribution with the mean μ=5,000 [days] and S. D. σ=2.200 [days]. In the case that these two units constituted a system in series, it was found with numerical simulations that the inspection interval which maximized the interval availability was about 100 days. And it was found that the interval availability became larger at most about 0. 02 if the exponential distribution was assumed as the failure distribution of the unit 2.