This paper deals with the research on improving the reliability of deep neural network (DNN) verifiers. Existing DNN verifiers have the problem of being vulnerable to errors and numerical inaccuracies, and attempts have been made to add a certificate generation function that enables independent algorithmic verification of the results to solve this problem. However, existing C++-based Marabou certificate verification implementations still have reliability issues (e.g., accuracy of floating-point calculations, soundness of implementation). In this paper, we present a method to obtain a complete proof of the certificate correctness by reimplementing Marabou certificate verification using Imandra, an industrial-purpose functional programming language and interactive theorem proverator (ITP). This provides stronger independent guarantees for Marabou proofs and paves the way for the widespread adoption of DNN verifiers in ITP.