A message relay system comprising: a plurality of client terminals (3,4); a line switching server configured to control a speech communication among a plurality of telephones (1,2); and a message relay server, wherein said message relay server comprises: a call state control section configured to acquire a call state data which contains identification data of said plurality of telephones (1,2) which perform a speech communication, from said line switching server; a call state storage section configured to store said call state data for every said speech communication; a message relay section configured to receive a message from a first client terminal of said plurality of client terminals which is in cooperation with a first telephone of said plurality of telephones, and specify a second telephone which performs the speech communication with said first telephone based on said call state data and the identification data of said first telephone in said message, and characterised by a message storage section configured to store said message and the identification data of said first and second telephones (1,2) as a message storage table and a reception state indicating whether said message has already been sent to said second telephone; wherein said message relay section is further configured to receive a message inquiry from a second client terminal of said plurality of client terminals which is in cooperation with said second telephone, transmits a message whose reception state is unsent among messages stored in said message storing section to said second client terminal based on the identification data of said second telephone in said message, and change the reception state of said message into sent. a plurality of client terminals (3,4); a line switching server configured to control a speech communication among a plurality of telephones (1,2); and a message relay server, a call state control section configured to acquire a call state data which contains identification data of said plurality of telephones (1,2) which perform a speech communication, from said line switching server; a call state storage section configured to store said call state data for every said speech communication; a message relay section configured to receive a message from a first client terminal of said plurality of client terminals which is in cooperation with a first telephone of said plurality of telephones, and specify a second telephone which performs the speech communication with said first telephone based on said call state data and the identification data of said first telephone in said message, and characterised by a message storage section configured to store said message and the identification data of said first and second telephones (1,2) as a message storage table and a reception state indicating whether said message has already been sent to said second telephone; wherein said message relay section is further configured to receive a message inquiry from a second client terminal of said plurality of client terminals which is in cooperation with said second telephone, transmits a message whose reception state is unsent among messages stored in said message storing section to said second client terminal based on the identification data of said second telephone in said message, and change the reception state of said message into sent.