A single-cylinder or multi-cylinder four-stroke engine unit (100, 200) comprising: a combustion chamber; an intake passage member through which air is introduced into the combustion chamber; and a throttle valve provided in an intermediate portion of the intake passage member, the combustion chamber, the intake passage member, and the throttle valve being provided for each cylinder, pressure in a downstream intake passage portion of the intake passage member that is downstream of the throttle valve varying in a pressure-variation manner such that: a smaller depression having a smaller difference from atmospheric pressure and a larger depression having a larger difference from atmospheric pressure are created in each four-stroke cycle constituted by four strokes; and the creation of the smaller and larger depressions is performed in each four-stroke cycle, the engine unit (100, 200) further comprising: a canister connected to a fuel tank and accommodating therein an adsorbent configured to adsorb fuel vapor contained in incoming air from the fuel tank; a communication passage member (163, 263) configured to establish communication between an inside of the canister and the downstream intake passage portion for each cylinder; a valve switchable from a closed state to an open state and switchable from the open state to the closed state, the closed state being the state where the valve prevents communication of air between the inside of the canister and the downstream intake passage portion, the open state being the state where the valve allows communication of air between the inside of the canister and the downstream intake passage portion; a controller configured to control the valve to perform a valve switching operation in association with the pressure-variation manner in which the creation of the smaller and larger depressions is performed in each four-stroke cycle, the valve switching operation being a set of switching-on and switching-off operations, one of the operations being performed first and then the other one of the operations being performed, the switching-on operation being an operation to switch the valve from the closed state to the open state, and the switching-off operation being an operation to switch the valve from the open state to the closed state; and an intake valve configured to open/close a communication opening between the downstream intake passage portion and the combustion chamber, characterized in that the controller is configured to control the valve so as to perform the switching-on operation at a timing within a last half of a closed period during which the intake valve is closed, and before the end of the closed period, perform the switching-off operation before the switching-on operation, during one four-stroke cycle in which the switching-on operation is performed, and perform each of the switching-on operation and the switching-off operation once in one four-stroke cycle in which the switching-on operation is performed. a canister connected to a fuel tank and accommodating therein an adsorbent configured to adsorb fuel vapor contained in incoming air from the fuel tank; a communication passage member (163, 263) configured to establish communication between an inside of the canister and the downstream intake passage portion for each cylinder; a valve switchable from a closed state to an open state and switchable from the open state to the closed state, the closed state being the state where the valve prevents communication of air between the inside of the canister and the downstream intake passage portion, the open state being the state where the valve allows communication of air between the inside of the canister and the downstream intake passage portion; a controller configured to control the valve to perform a valve switching operation in association with the pressure-variation manner in which the creation of the smaller and larger depressions is performed in each four-stroke cycle, the valve switching operation being a set of switching-on and switching-off operations, one of the operations being performed first and then the other one of the operations being performed, the switching-on operation being an operation to switch the valve from the closed state to the open state, and the switching-off operation being an operation to switch the valve from the open state to the closed state; and an intake valve configured to open/close a communication opening between the downstream intake passage portion and the combustion chamber, characterized in that the controller is configured to control the valve so as to perform the switching-on operation at a timing within a last half of a closed period during which the intake valve is closed, and before the end of the closed period, perform the switching-off operation before the switching-on operation, during one four-stroke cycle in which the switching-on operation is performed, and perform each of the switching-on operation and the switching-off operation once in one four-stroke cycle in which the switching-on operation is performed. control the valve so as to perform the switching-on operation at a timing within a last half of a closed period during which the intake valve is closed, and before the end of the closed period, perform the switching-off operation before the switching-on operation, during one four-stroke cycle in which the switching-on operation is performed, and perform each of the switching-on operation and the switching-off operation once in one four-stroke cycle in which the switching-on operation is performed.