A magnetic field application device (1D to 1F) applying a magnetic field to a magnetocaloric material, the magnetic field application device (1D to 1F) comprising: a magnetic field generating component; a first yoke (11, 11F) connected to both poles of the magnetic field generating component; and a second yoke (12, 12F) disposed movably between a first position and a second position; wherein a first closed magnetic circuit formed by the magnetic field generating component and the first yoke (11, 11F) passes through a magnetic field application region in which the magnetocaloric material is accommodated, a second closed magnetic circuit formed by the magnetic field generating component, the first yoke (11, 11F), and the second yoke (12, 12F) bypasses the magnetic field application region, a relative positional relationship among the magnetic field generating component, the first yoke (11, 11F), and the magnetic field application region is constant, and magnetic resistance of the second closed magnetic circuit when the second yoke (12, 12F) is located at the second position is smaller than the magnetic resistance of the second closed magnetic circuit when the second yoke (12, 12F) is located at the first position, characterized in that the second closed magnetic circuit is larger than the first closed magnetic circuit, the second yoke (12, 12F) moves from the first position to the second position by increasing magnetic force generated between the second yoke (12, 12F) and the first yoke (11, 11F), the magnetic field generating component is an electromagnet, the magnetic field application device (1D to 1F) further comprises a controller (15D, 15E) to control magnetic force of the electromagnet, the magnetic force generated between the first yoke (11, 11F) and the second yoke (12, 12F) is increased when the controller (15D, 15E) increases the magnetic force of the electromagnet, the controller (15D, 15E) periodically changes the magnetic force of the electromagnet and the position of the second yoke (12, 12F) in order of a first state, a second state, a third state, and a fourth state, the first state is a state in which the magnetic force of the electromagnet has first intensity and the second yoke (12, 12F) is located at the first position, the second state is a state in which the magnetic force of the electromagnet has second intensity larger than the first intensity and the second yoke (12, 12F) is located at the first position, the third state is a state in which the magnetic force of the electromagnet has the second intensity and the second yoke (12, 12F) is located at the second position, and the fourth state is a state in which the magnetic force of the electromagnet has the first intensity and the second yoke (12, 12F) is located at the second position. a magnetic field generating component; a first yoke (11, 11F) connected to both poles of the magnetic field generating component; and a second yoke (12, 12F) disposed movably between a first position and a second position; wherein a first closed magnetic circuit formed by the magnetic field generating component and the first yoke (11, 11F) passes through a magnetic field application region in which the magnetocaloric material is accommodated, a second closed magnetic circuit formed by the magnetic field generating component, the first yoke (11, 11F), and the second yoke (12, 12F) bypasses the magnetic field application region, a relative positional relationship among the magnetic field generating component, the first yoke (11, 11F), and the magnetic field application region is constant, and magnetic resistance of the second closed magnetic circuit when the second yoke (12, 12F) is located at the second position is smaller than the magnetic resistance of the second closed magnetic circuit when the second yoke (12, 12F) is located at the first position, characterized in that the second closed magnetic circuit is larger than the first closed magnetic circuit, the second yoke (12, 12F) moves from the first position to the second position by increasing magnetic force generated between the second yoke (12, 12F) and the first yoke (11, 11F), the magnetic field generating component is an electromagnet, the magnetic field application device (1D to 1F) further comprises a controller (15D, 15E) to control magnetic force of the electromagnet, the magnetic force generated between the first yoke (11, 11F) and the second yoke (12, 12F) is increased when the controller (15D, 15E) increases the magnetic force of the electromagnet, the controller (15D, 15E) periodically changes the magnetic force of the electromagnet and the position of the second yoke (12, 12F) in order of a first state, a second state, a third state, and a fourth state, the first state is a state in which the magnetic force of the electromagnet has first intensity and the second yoke (12, 12F) is located at the first position, the second state is a state in which the magnetic force of the electromagnet has second intensity larger than the first intensity and the second yoke (12, 12F) is located at the first position, the third state is a state in which the magnetic force of the electromagnet has the second intensity and the second yoke (12, 12F) is located at the second position, and the fourth state is a state in which the magnetic force of the electromagnet has the first intensity and the second yoke (12, 12F) is located at the second position.