This paper presents Automatic Gain Control (AGC) algorithm in maritime communication system using application specific message (ASM). In maritime communication, gain control is not easy than land mobile communication. Received signal is converted to digital signal by analog to digital converter (ADC). If the signal input power of ADC is smaller than adequate power then quantization noise increases. On the contrary, if the signal input power of ADC is large than adequate power then the output signal of ADC is saturated. This paper proposes essential algorithm that controls gain value effectively in case that received signal power and propagation delay are unknown. We first analyze the general method of AGC in maritime communication. The analysis will show that the time for gain convergence is needed and much longer than known case because the distance from transmitting ship to receiving ship is randomly distributed and unknown in each slot. The difference of gain values in each slot is as large as 100dB in maximum case. The larger a difference of gain values is, the longer gain convergence time is. This is because that any ship can transmit in any slot randomly, therefore, received signal power is not expectable. We show how proposed algorithm can effectively control the gain value in maritime communication environments. Result of simulation shows that the utilization of proposed algorithm can calculate the optimal gain value and regenerate optimal signal that is similar with original source. As a result, proposed AGC algorithm changes all gain values of total received signal into same gain value using back gain compensation. Especially, the gain value of received signal should be controlled in adequate because transmission randomly occurs in each slot in maritime communication. Proposed automatic gain control algorithm is essential and effective for stable demodulation performance in ASM maritime communication.