A coolant coupling device (20) includes: a cylinder chamber (25) provided for a support member (2) for a tool rest (4) in a machine tool and supplied with coolant; a piston (P) movably arranged in the cylinder chamber (25); and a coolant supply flow channel (30) provided for the piston (P) for passing coolant therethrough. The piston (P) is driven by the coolant supplied to the cylinder chamber (25) so that an outlet (32) of the coolant supply flow channel (30) is connected to a coolant inlet port (10) of the tool rest (4) to supply the coolant to the tool rest (4). The piston (P) comprises a cylindrical projection (29) protruding toward and into the cylinder chamber (25), and the coolant supply flow channel (30) has an inlet (34) that opens at an outer peripheral surface of the cylindrical projection (29).