A master cylinder apparatus comprising: an input piston that is configured to move forward by operating a brake operating member; a pressure piston that is provided in front of the input piston and configured to move relative to the input piston; and a stroke velocity ratio modification device (15, 25, 22, 200) configured to modify a stroke velocity ratio, which is a ratio between a stroke velocity of the pressure piston and a stroke velocity of the input piston, in at least two stages within a range not greater than a predetermined value larger than 1 while the input piston moves from a retreat end position to a advancement end position, wherein the input piston is disposed opposite the pressure piston via an inter-piston chamber, and the pressure piston includes a large diameter portion, and a front small diameter portion that is provided in front of the large diameter portion and has a smaller diameter than the large diameter portion, characterized in that a surface area of the large diameter portion of the pressure piston on which pressure is received from a front side is smaller than a surface area of the pressure piston on which pressure is received from the inter-piston chamber side, wherein a step constituted by the large diameter portion and the front small diameter portion, and the stroke velocity ratio modification device (15, 25, 22, 200) includes a communication condition control device provided between an opposing chamber, which is provided in front of the step, the inter-piston chamber and a reservoir, the communication condition control device being configured to control communication conditions therebetween, wherein the communication condition control device being configured to switch between an inter-chamber connection condition in which the opposing chamber and the inter-piston chamber communicate with each other but are cut off from the reservoir, and an inter-chamber cutoff condition in which the opposing chamber is cut off from the inter-piston chamber, the inter-piston chamber is cut off from the reservoir, and the opposing chamber communicates with the reservoir.. an input piston that is configured to move forward by operating a brake operating member; a pressure piston that is provided in front of the input piston and configured to move relative to the input piston; and a stroke velocity ratio modification device (15, 25, 22, 200) configured to modify a stroke velocity ratio, which is a ratio between a stroke velocity of the pressure piston and a stroke velocity of the input piston, in at least two stages within a range not greater than a predetermined value larger than 1 while the input piston moves from a retreat end position to a advancement end position, wherein the input piston is disposed opposite the pressure piston via an inter-piston chamber, and the pressure piston includes a large diameter portion, and a front small diameter portion that is provided in front of the large diameter portion and has a smaller diameter than the large diameter portion, characterized in that a surface area of the large diameter portion of the pressure piston on which pressure is received from a front side is smaller than a surface area of the pressure piston on which pressure is received from the inter-piston chamber side, wherein a step constituted by the large diameter portion and the front small diameter portion, and the stroke velocity ratio modification device (15, 25, 22, 200) includes a communication condition control device provided between an opposing chamber, which is provided in front of the step, the inter-piston chamber and a reservoir, the communication condition control device being configured to control communication conditions therebetween, wherein the communication condition control device being configured to switch between an inter-chamber connection condition in which the opposing chamber and the inter-piston chamber communicate with each other but are cut off from the reservoir, and an inter-chamber cutoff condition in which the opposing chamber is cut off from the inter-piston chamber, the inter-piston chamber is cut off from the reservoir, and the opposing chamber communicates with the reservoir..