Provided is a tool tight-contact-state detector for a machine tool which is capable of accurately and stably detecting whether or not a tool or a tool holder having the tool attached thereto is mounted to a main spindle in tight contact therewith. While rotation of a main spindle (13) is stopped, hydraulic pressure is supplied from a hydraulic pressure supply unit (28) to a cylinder section (20) so as to bring a piston member (21) to into contact with a main spindle head (11) and the main spindle (13) against a biasing force from an O-ring (22). Air is thereby allowed to be supplied to air ejection holes (27a, 27b, 27c), which are formed in a forward end face (13a) of the main spindle (13), from the main spindle head (11) side through the piston member (21). Then, a judgment is made as to whether or not an end face (41) of a tool (2) and a forward end face (13a) of the main spindle (13) are in tight contact therewith, on the basis of detection by air pressure detection sensors (25a, 25b, 25c) regarding pressure variations in the air injected from the air ejection holes (27a, 27b, 27c).