A spool valve assembly for switching a state of a hydraulic machine which includes a hydraulic chamber, a high pressure oil line, and a low pressure oil line, between a first state where the hydraulic chamber is in communication with the high pressure oil line and a second state where the hydraulic chamber is in communication with the low pressure oil line, wherein the spool valve assembly comprises a first spool valve which comprises: a first chamber; a second chamber; and a first spool including a first pressure-receiving surface formed so as to receive hydraulic pressure of the first chamber and a second pressure-receiving surface formed so as to receive hydraulic pressure of the second chamber, the second pressure-receiving surface having an area smaller than that of the first pressure-receiving surface, wherein the first spool valve is configured to switch a communication state of the hydraulic chamber between the first state and the second state, by switching the hydraulic pressure of the first chamber between the pressure in the high pressure oil line and a first pressure which is lower than the pressure in the high pressure oil line while maintaining the hydraulic pressure in the second chamber at the pressure of the high pressure oil line, or by switching the hydraulic pressure of the second chamber between the pressure in the low pressure oil line and a second pressure which is higher than the pressure in the low pressure oil line while maintaining the hydraulic pressure in the first chamber at the pressure of the low pressure oil line, wherein the first spool valve comprises a casing which accommodates the first spool, wherein the first spool comprises a land portion (56, 58) and a groove portion which is disposed adjacent to the land portion (56, 58), the first spool being configured to be movable in an axial direction of the first spool, wherein the casing comprises a high pressure communication path configured to communicate with the high pressure oil line, a low pressure communication path configured to communicate with the low pressure oil line, and a hydraulic chamber communication path configured to communicate with the hydraulic chamber, wherein the first state is a state where the high pressure communication path is in communication with the hydraulic chamber communication path via the groove portion, and wherein the second state is a state where the low pressure communication path is in communication with the hydraulic chamber communication path via the groove portion, and wherein the first spool valve includes a first drain chamber formed at one end side of the first spool and a second drain chamber formed at other end side of the first spool in the axial direction of the first spool, wherein the first drain chamber and the second drain chamber are configured to communicate with each other via a first drain flow path disposed in the casing and/or a second drain flow path disposed in the first spool, wherein the first spool valve includes an end wall on one end side of the casing in the axial direction of the first spool, characterized in that the first drain chamber is formed between the end wall and the first spool, and wherein the end wall includes a cut-out through which the first drain chamber is configured to communicate with the first drain flow path. a first chamber; a second chamber; and a first spool including a first pressure-receiving surface formed so as to receive hydraulic pressure of the first chamber and a second pressure-receiving surface formed so as to receive hydraulic pressure of the second chamber, the second pressure-receiving surface having an area smaller than that of the first pressure-receiving surface,