With an increase in the proportion of software in systems used in each industry, software safety-related incidents are increasing. To solve this issue, major industrial sectors such as railways, aerospace and medical services are working to secure software safety in accordance with IEC 61508-based functional safety standard. However, since the standard document only presents a list of outputs and declarative sentences, it is difficult to directly apply it practical business affairs, In addition, hardware-centered functional safety standards are difficult to apply to software. The common development guide for software safety published by the National IT Industry Promotion Agency in 2016 provides common guidelines that can be utilized in various fields based on the IEC 61508 standard, but additional customizing is required to apply to a specific field or scale. In this regard, this paper attempted to define processes that can be applied to small-scale software and outputs of each stage and to apply the software safety process to an echo sounder, which is a device for marine communications, as an example of the maritime sector.