This master cylinder device is provided with: a housing (150) closed at the front end and having a third housing member (164) which partitions the inside of said housing into a front chamber (R1) and a rear chamber (R2) and which has an opening passing through said third housing member (164); a first pressurizing piston (152) having a main body (180) arranged in the front chamber and moving forward upon receiving the force for pressurizing the working fluid supplied to a brake device (56); and an input piston (156) capable of moving forward through an operation force applied to a brake pedal (70). The master cylinder device is characterised by an input chamber (R5) and an inter-piston chamber (R8) being formed between the third housing member and the rear end of the main body of the first pressurizing piston (152). A working fluid is introduced into the input chamber (R5) from a high-pressure source device (58). In the inter-piston chamber (R8), by means of the input piston being seal-fitted with the housing, an opening formed in the partition part between the pistons is utilized to allow the input piston and the pressure-receiving piston to face one another without the input piston and the pressure-receiving piston being seal-fitted.