A robot arm coupling device comprising a master plate provided at a leading end portion of a robot arm, a tool plate that can be connected to the master plate and to which a tool is attached, and a locking mechanism that detachably connects the tool plate to the master plate, wherein: the locking mechanism comprises: a concave engagement portion that is formed on a lower side portion of the master plate so as to open downward; a convex engagement portion that is formed on an upper side portion of the tool plate and is capable of being inserted into the concave engagement portion; a plurality of engagement members that are installed at an external circumferential wall portion of the concave engagement portion, and are capable of being changed over between locking positions in which they engage with an engagement portion formed on the convex engagement portion, and unlocking positions in which they do not engage with the engagement portion; and a fluid pressure cylinder (20, 20A) that is formed in the master plate, and is capable of changing over the plurality of engagement members between unlocking positions and locking positions; and wherein at least one fluid pressure passage (23, 24) that supplies and/or discharges fluid to and/or from the fluid pressure cylinder is provided in the master plate, more radially inward than the plurality of engagement members and the fluid pressure cylinder. the locking mechanism comprises: a concave engagement portion that is formed on a lower side portion of the master plate so as to open downward; a convex engagement portion that is formed on an upper side portion of the tool plate and is capable of being inserted into the concave engagement portion; a plurality of engagement members that are installed at an external circumferential wall portion of the concave engagement portion, and are capable of being changed over between locking positions in which they engage with an engagement portion formed on the convex engagement portion, and unlocking positions in which they do not engage with the engagement portion; and a fluid pressure cylinder (20, 20A) that is formed in the master plate, and is capable of changing over the plurality of engagement members between unlocking positions and locking positions; a concave engagement portion that is formed on a lower side portion of the master plate so as to open downward; a convex engagement portion that is formed on an upper side portion of the tool plate and is capable of being inserted into the concave engagement portion; a plurality of engagement members that are installed at an external circumferential wall portion of the concave engagement portion, and are capable of being changed over between locking positions in which they engage with an engagement portion formed on the convex engagement portion, and unlocking positions in which they do not engage with the engagement portion; and a fluid pressure cylinder (20, 20A) that is formed in the master plate, and is capable of changing over the plurality of engagement members between unlocking positions and locking positions; and wherein at least one fluid pressure passage (23, 24) that supplies and/or discharges fluid to and/or from the fluid pressure cylinder is provided in the master plate, more radially inward than the plurality of engagement members and the fluid pressure cylinder.