A ballast water treatment device comprising: a bypass pipe having both ends connected to a ballast pipe which is connected to a ballast tank and through which ballast water flows, the bypass pipe being configured to allow a part of ballast water flowing through the ballast pipe to form a divided flow from the ballast pipe and to allow the divided flow to be merged with the ballast pipe; a chemical holding part in which a chemical is stored; and a chemical pipe configured to connect the chemical holding part and the bypass pipe with each other and supply the chemical from the chemical holding part to the bypass pipe; wherein the chemical holding part is a sterilizing agent holding part in which a sterilizing agent as the chemical is stored; wherein the chemical pipe is a sterilization pipe which connects the sterilizing agent holding part and the bypass pipe with each other, and supplies the sterilizing agent from the sterilizing agent holding part to the bypass pipe such that the chemical held by the chemical holding part is diluted once by ballast water which is divided from the ballast pipe and flows through the bypass pipe, and thereafter is supplied to the ballast pipe; and wherein the ballast water treatment device further comprises: a neutralizing agent holding part in which a neutralizing agent having a property of neutralizing the sterilizing agent is stored; and a neutralization pipe which connects the neutralizing agent holding part and the bypass pipe with each other, and supplies the neutralizing agent from the neutralizing agent holding part to the bypass pipe, wherein a connecting portion between the neutralization pipe and the bypass pipe is positioned on an upstream side of the bypass pipe, compared to a connecting portion between the sterilization pipe and the bypass pipe, and wherein at least an inner surface of the bypass pipe is formed using a material having more excellent corrosion resistance than a material used for forming the ballast pipe. a bypass pipe having both ends connected to a ballast pipe which is connected to a ballast tank and through which ballast water flows, the bypass pipe being configured to allow a part of ballast water flowing through the ballast pipe to form a divided flow from the ballast pipe and to allow the divided flow to be merged with the ballast pipe; a chemical holding part in which a chemical is stored; and a chemical pipe configured to connect the chemical holding part and the bypass pipe with each other and supply the chemical from the chemical holding part to the bypass pipe; wherein the chemical holding part is a sterilizing agent holding part in which a sterilizing agent as the chemical is stored; wherein the chemical pipe is a sterilization pipe which connects the sterilizing agent holding part and the bypass pipe with each other, and supplies the sterilizing agent from the sterilizing agent holding part to the bypass pipe such that the chemical held by the chemical holding part is diluted once by ballast water which is divided from the ballast pipe and flows through the bypass pipe, and thereafter is supplied to the ballast pipe; and wherein the ballast water treatment device further comprises: a neutralizing agent holding part in which a neutralizing agent having a property of neutralizing the sterilizing agent is stored; and a neutralization pipe which connects the neutralizing agent holding part and the bypass pipe with each other, and supplies the neutralizing agent from the neutralizing agent holding part to the bypass pipe, a neutralizing agent holding part in which a neutralizing agent having a property of neutralizing the sterilizing agent is stored; and a neutralization pipe which connects the neutralizing agent holding part and the bypass pipe with each other, and supplies the neutralizing agent from the neutralizing agent holding part to the bypass pipe, wherein a connecting portion between the neutralization pipe and the bypass pipe is positioned on an upstream side of the bypass pipe, compared to a connecting portion between the sterilization pipe and the bypass pipe, and wherein at least an inner surface of the bypass pipe is formed using a material having more excellent corrosion resistance than a material used for forming the ballast pipe.